build_web_compilers/lib/src/dev_compiler_bootstrap.dart - third_party/dart-pkg - Git at Google

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.

 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';

 import 'package:build/build.dart';
 import 'package:build_modules/build_modules.dart';
 import 'package:meta/meta.dart';
 import 'package:path/path.dart' as _p; // ignore: library_prefixes
 import 'package:pool/pool.dart';

 import 'ddc_names.dart';
 import 'dev_compiler_builder.dart';
 import 'platforms.dart';
 import 'web_entrypoint_builder.dart';

 /// Alias `_p.url` to `p`.
 _p.Context get _context => _p.url;

 var _modulePartialExtension = _context.withoutExtension(jsModuleExtension);

 /// Bootstraps a ddc application, creating the main entrypoint as well as the
 /// bootstrap and digest entrypoints.
 ///
 /// If [skipPlatformCheck] is `true` then all `dart:` imports will be
 /// allowed in all packages.
 ///
 /// Deprecated: If [skipPlatformCheckPackages] is provided then any dart:
 /// imports will be allowed in the specified packages.
 ///
 /// If [requiredAssets] is provided then this will ensure those assets are
 /// available to the app by making them inputs of this build action.
 Future<void> bootstrapDdc(
   BuildStep buildStep, {
   DartPlatform platform,
   bool skipPlatformCheck = false,
   @deprecated Set<String> skipPlatformCheckPackages = const {},
   Iterable<AssetId> requiredAssets,
 }) async {
   requiredAssets ??= [];
   skipPlatformCheck ??= false;
   // Ensures that the sdk resources are built and available.
   await _ensureResources(buildStep, requiredAssets);

   var dartEntrypointId = buildStep.inputId;
   var moduleId = buildStep.inputId
       .changeExtension(moduleExtension(platform ?? ddcPlatform));
   var module = Module.fromJson(json
       .decode(await buildStep.readAsString(moduleId)) as Map<String, dynamic>);

   // First, ensure all transitive modules are built.
   List<AssetId> transitiveJsModules;
   try {
     transitiveJsModules = await _ensureTransitiveJsModules(module, buildStep,
         skipPlatformCheck: skipPlatformCheck,
         skipPlatformCheckPackages: skipPlatformCheckPackages);
   } on UnsupportedModules catch (e) {
     var librariesString = (await e.exactLibraries(buildStep).toList())
         .map((lib) => AssetId(lib.id.package,
             lib.id.path.replaceFirst(moduleLibraryExtension, '.dart')))
         .join('\n');
     log.warning('''
 Skipping compiling ${buildStep.inputId} with ddc because some of its
 transitive libraries have sdk dependencies that not supported on this platform:

 $librariesString

 https://github.com/dart-lang/build/blob/master/docs/faq.md#how-can-i-resolve-skipped-compiling-warnings
 ''');
     return;
   }
   var jsId = module.primarySource.changeExtension(jsModuleExtension);
   var appModuleName = ddcModuleName(jsId);
   var appDigestsOutput =
       dartEntrypointId.changeExtension(digestsEntrypointExtension);

   // The name of the entrypoint dart library within the entrypoint JS module.
   //
   // This is used to invoke `main()` from within the bootstrap script.
   //
   // TODO(jakemac53): Sane module name creation, this only works in the most
   // basic of cases.
   //
   // See https://github.com/dart-lang/sdk/issues/27262 for the root issue
   // which will allow us to not rely on the naming schemes that dartdevc uses
   // internally, but instead specify our own.
   var oldAppModuleScope = toJSIdentifier(
       _context.withoutExtension(_context.basename(buildStep.inputId.path)));

   // Like above but with a package-relative entrypoint.
   var appModuleScope =
       pathToJSIdentifier(_context.withoutExtension(buildStep.inputId.path));

   // Map from module name to module path for custom modules.
   var modulePaths = SplayTreeMap.of(
       {'dart_sdk': r'packages/build_web_compilers/src/dev_compiler/dart_sdk'});
   for (var jsId in transitiveJsModules) {
     // Strip out the top level dir from the path for any module, and set it to
     // `packages/` for lib modules. We set baseUrl to `/` to simplify things,
     // and we only allow you to serve top level directories.
     var moduleName = ddcModuleName(jsId);
     modulePaths[moduleName] = _context.withoutExtension(
         jsId.path.startsWith('lib')
             ? '$moduleName$jsModuleExtension'
             : _context.joinAll(_context.split(jsId.path).skip(1)));
   }

   var bootstrapId = dartEntrypointId.changeExtension(ddcBootstrapExtension);
   var bootstrapModuleName = _context.withoutExtension(_context.relative(
       bootstrapId.path,
       from: _context.dirname(dartEntrypointId.path)));

   var dartEntrypointParts = _context.split(dartEntrypointId.path);
   var entrypointLibraryName = _context.joinAll([
     // Convert to a package: uri for files under lib.
     if (dartEntrypointParts.first == 'lib')
       'package:${module.primarySource.package}',
     // Strip top-level directory from the path.
     ...dartEntrypointParts.skip(1),
   ]);

   var bootstrapContent =
       StringBuffer('$_entrypointExtensionMarker\n(function() {\n')
         ..write(_dartLoaderSetup(
             modulePaths,
             _p.url.relative(appDigestsOutput.path,
                 from: _p.url.dirname(bootstrapId.path))))
         ..write(_requireJsConfig)
         ..write(_appBootstrap(bootstrapModuleName, appModuleName,
             appModuleScope, entrypointLibraryName,
             oldModuleScope: oldAppModuleScope));

   await buildStep.writeAsString(bootstrapId, bootstrapContent.toString());

   var entrypointJsContent = _entryPointJs(bootstrapModuleName);
   await buildStep.writeAsString(
       dartEntrypointId.changeExtension(jsEntrypointExtension),
       entrypointJsContent);

   // Output the digests for transitive modules.
   // These can be consumed for hot reloads.
   var moduleDigests = <String, String>{
     for (var jsId in transitiveJsModules)
       _moduleDigestKey(jsId): '${await buildStep.digest(jsId)}',
   };
   await buildStep.writeAsString(appDigestsOutput, jsonEncode(moduleDigests));
 }

 String _moduleDigestKey(AssetId jsId) =>
     '${ddcModuleName(jsId)}$jsModuleExtension';

 final _lazyBuildPool = Pool(16);

 /// Ensures that all transitive js modules for [module] are available and built.
 ///
 /// Throws an [UnsupportedModules] exception if there are any
 /// unsupported modules.
 Future<List<AssetId>> _ensureTransitiveJsModules(
     Module module, BuildStep buildStep,
     {@required bool skipPlatformCheck,
     @required Set<String> skipPlatformCheckPackages}) async {
   // Collect all the modules this module depends on, plus this module.
   var transitiveDeps = await module.computeTransitiveDependencies(buildStep,
       throwIfUnsupported: !skipPlatformCheck,
       // ignore: deprecated_member_use
       skipPlatformCheckPackages: skipPlatformCheckPackages);

   var jsModules = [
     module.primarySource.changeExtension(jsModuleExtension),
     for (var dep in transitiveDeps)
       dep.primarySource.changeExtension(jsModuleExtension),
   ];
   // Check that each module is readable, and warn otherwise.
   await Future.wait(jsModules.map((jsId) async {
     if (await _lazyBuildPool.withResource(() => buildStep.canRead(jsId))) {
       return;
     }
     var errorsId = jsId.addExtension('.errors');
     await buildStep.canRead(errorsId);
     log.warning('Unable to read $jsId, check your console or the '
         '`.dart_tool/build/generated/${errorsId.package}/${errorsId.path}` '
         'log file.');
   }));
   return jsModules;
 }

 /// Code that actually imports the [moduleName] module, and calls the
 /// `[moduleScope].main()` function on it.
 ///
 /// Also performs other necessary initialization.
 String _appBootstrap(String bootstrapModuleName, String moduleName,
         String moduleScope, String entrypointLibraryName,
         {String oldModuleScope}) =>
     '''
 define("$bootstrapModuleName", ["$moduleName", "dart_sdk"], function(app, dart_sdk) {
   dart_sdk.dart.setStartAsyncSynchronously(true);
   dart_sdk._isolate_helper.startRootIsolate(() => {}, []);
   $_initializeTools
   $_mainExtensionMarker
   (app.$moduleScope || app.$oldModuleScope).main();
   var bootstrap = {
       hot\$onChildUpdate: function(childName, child) {
         // Special handling for the multi-root scheme uris. We need to strip
         // out the scheme and the top level directory, to match the source path
         // that chrome sees.
         if (childName.startsWith('$multiRootScheme:///')) {
           childName = childName.substring('$multiRootScheme:///'.length);
           var firstSlash = childName.indexOf('/');
           if (firstSlash == -1) return false;
           childName = childName.substring(firstSlash + 1);
         }
         if (childName === "$entrypointLibraryName") {
           // Clear static caches.
           dart_sdk.dart.hotRestart();
           child.main();
           return true;
         }
       }
     }
   dart_sdk.dart.trackLibraries("$bootstrapModuleName", {
     "$bootstrapModuleName": bootstrap
   }, '');
   return {
     bootstrap: bootstrap
   };
 });
 })();
 ''';

 /// The actual entrypoint JS file which injects all the necessary scripts to
 /// run the app.
 String _entryPointJs(String bootstrapModuleName) => '''
 (function() {
   $_currentDirectoryScript
   $_baseUrlScript

   var mapperUri = baseUrl + "packages/build_web_compilers/src/" +
       "dev_compiler_stack_trace/stack_trace_mapper.dart.js";
   var requireUri = baseUrl +
       "packages/build_web_compilers/src/dev_compiler/require.js";
   var mainUri = _currentDirectory + "$bootstrapModuleName";

   if (typeof document != 'undefined') {
     var el = document.createElement("script");
     el.defer = true;
     el.async = false;
     el.src = mapperUri;
     document.head.appendChild(el);

     el = document.createElement("script");
     el.defer = true;
     el.async = false;
     el.src = requireUri;
     el.setAttribute("data-main", mainUri);
     document.head.appendChild(el);
   } else {
     importScripts(mapperUri, requireUri);
     require.config({
       baseUrl: baseUrl,
     });
     // TODO: update bootstrap code to take argument - dart-lang/build#1115
     window = self;
     require([mainUri + '.js']);
   }
 })();
 ''';

 /// JavaScript snippet to determine the directory a script was run from.
 final _currentDirectoryScript = r'''
 var _currentDirectory = (function () {
   var _url;
   var lines = new Error().stack.split('\n');
   function lookupUrl() {
     if (lines.length > 2) {
       var match = lines[1].match(/^\s+at (.+):\d+:\d+$/);
       // Chrome.
       if (match) return match[1];
       // Chrome nested eval case.
       match = lines[1].match(/^\s+at eval [(](.+):\d+:\d+[)]$/);
       if (match) return match[1];
       // Edge.
       match = lines[1].match(/^\s+at.+\((.+):\d+:\d+\)$/);
       if (match) return match[1];
       // Firefox.
       match = lines[0].match(/[<][@](.+):\d+:\d+$/)
       if (match) return match[1];
     }
     // Safari.
     return lines[0].match(/(.+):\d+:\d+$/)[1];
   }
   _url = lookupUrl();
   var lastSlash = _url.lastIndexOf('/');
   if (lastSlash == -1) return _url;
   var currentDirectory = _url.substring(0, lastSlash + 1);
   return currentDirectory;
 })();
 ''';

 /// Sets up `window.$dartLoader` based on [modulePaths].
 String _dartLoaderSetup(Map<String, String> modulePaths, String appDigests) =>
     '''
 $_currentDirectoryScript
 $_baseUrlScript
 let modulePaths = ${const JsonEncoder.withIndent(" ").convert(modulePaths)};
 if(!window.\$dartLoader) {
    window.\$dartLoader = {
      appDigests: _currentDirectory + '$appDigests',
      moduleIdToUrl: new Map(),
      urlToModuleId: new Map(),
      rootDirectories: new Array(),
      // Used in package:build_runner/src/server/build_updates_client/hot_reload_client.dart
      moduleParentsGraph: new Map(),
      moduleLoadingErrorCallbacks: new Map(),
      forceLoadModule: function (moduleName, callback, onError) {
        // dartdevc only strips the final extension when adding modules to source
        // maps, so we need to do the same.
        if (moduleName.endsWith('$_modulePartialExtension')) {
          moduleName = moduleName.substring(0, moduleName.length - ${_modulePartialExtension.length});
        }
        if (typeof onError != 'undefined') {
          var errorCallbacks = \$dartLoader.moduleLoadingErrorCallbacks;
          if (!errorCallbacks.has(moduleName)) {
            errorCallbacks.set(moduleName, new Set());
          }
          errorCallbacks.get(moduleName).add(onError);
        }
        requirejs.undef(moduleName);
        requirejs([moduleName], function() {
          if (typeof onError != 'undefined') {
            errorCallbacks.get(moduleName).delete(onError);
          }
          if (typeof callback != 'undefined') {
            callback();
          }
        });
      },
      getModuleLibraries: null, // set up by _initializeTools
    };
 }
 let customModulePaths = {};
 window.\$dartLoader.rootDirectories.push(window.location.origin + baseUrl);
 for (let moduleName of Object.getOwnPropertyNames(modulePaths)) {
   let modulePath = modulePaths[moduleName];
   if (modulePath != moduleName) {
     customModulePaths[moduleName] = modulePath;
   }
   var src = window.location.origin + '/' + modulePath + '.js';
   if (window.\$dartLoader.moduleIdToUrl.has(moduleName)) {
     continue;
   }
   \$dartLoader.moduleIdToUrl.set(moduleName, src);
   \$dartLoader.urlToModuleId.set(src, moduleName);
 }
 ''';

 /// Code to initialize the dev tools formatter, stack trace mapper, and any
 /// other tools.
 ///
 /// Posts a message to the window when done.
 final _initializeTools = '''
 $_baseUrlScript
   dart_sdk._debugger.registerDevtoolsFormatter();
   \$dartLoader.getModuleLibraries = dart_sdk.dart.getModuleLibraries;
   if (window.\$dartStackTraceUtility && !window.\$dartStackTraceUtility.ready) {
     window.\$dartStackTraceUtility.ready = true;
     let dart = dart_sdk.dart;
     window.\$dartStackTraceUtility.setSourceMapProvider(
       function(url) {
         url = url.replace(baseUrl, '/');
         var module = window.\$dartLoader.urlToModuleId.get(url);
         if (!module) return null;
         return dart.getSourceMap(module);
       });
   }
   if (typeof document != 'undefined') {
     window.postMessage({ type: "DDC_STATE_CHANGE", state: "start" }, "*");
   }
 ''';

 /// Require JS config for ddc.
 ///
 /// Sets the base url to `/` so that all modules can be loaded using absolute
 /// paths which simplifies a lot of scenarios.
 ///
 /// Sets the timeout for loading modules to infinity (0).
 ///
 /// Sets up the custom module paths.
 ///
 /// Adds error handler code for require.js which requests a `.errors` file for
 /// any failed module, and logs it to the console.
 final _requireJsConfig = '''
 // Whenever we fail to load a JS module, try to request the corresponding
 // `.errors` file, and log it to the console.
 (function() {
   var oldOnError = requirejs.onError;
   requirejs.onError = function(e) {
     if (e.requireModules) {
       if (e.message) {
         // If error occurred on loading dependencies, we need to invalidate ancessor too.
         var ancesor = e.message.match(/needed by: (.*)/);
         if (ancesor) {
           e.requireModules.push(ancesor[1]);
         }
       }
       for (const module of e.requireModules) {
         var errorCallbacks = \$dartLoader.moduleLoadingErrorCallbacks.get(module);
         if (errorCallbacks) {
           for (const callback of errorCallbacks) callback(e);
           errorCallbacks.clear();
         }
       }
     }
     if (e.originalError && e.originalError.srcElement) {
       var xhr = new XMLHttpRequest();
       xhr.onreadystatechange = function() {
         if (this.readyState == 4) {
           var message;
           if (this.status == 200) {
             message = this.responseText;
           } else {
             message = "Unknown error loading " + e.originalError.srcElement.src;
           }
           console.error(message);
           var errorEvent = new CustomEvent(
             'dartLoadException', { detail: message });
           window.dispatchEvent(errorEvent);
         }
       };
       xhr.open("GET", e.originalError.srcElement.src + ".errors", true);
       xhr.send();
     }
     // Also handle errors the normal way.
     if (oldOnError) oldOnError(e);
   };
 }());

 $_baseUrlScript;

 require.config({
     baseUrl: baseUrl,
     waitSeconds: 0,
     paths: customModulePaths
 });

 const modulesGraph = new Map();
 function getRegisteredModuleName(moduleMap) {
   if (\$dartLoader.moduleIdToUrl.has(moduleMap.name + '$_modulePartialExtension')) {
     return moduleMap.name + '$_modulePartialExtension';
   }
   return moduleMap.name;
 }
 requirejs.onResourceLoad = function (context, map, depArray) {
   const name = getRegisteredModuleName(map);
   const depNameArray = depArray.map(getRegisteredModuleName);
   if (modulesGraph.has(name)) {
     // TODO Move this logic to better place
     var previousDeps = modulesGraph.get(name);
     var changed = previousDeps.length != depNameArray.length;
     changed = changed || depNameArray.some(function(depName) {
       return !previousDeps.includes(depName);
     });
     if (changed) {
       console.warn("Dependencies graph change for module '" + name + "' detected. " +
         "Dependencies was [" + previousDeps + "], now [" +  depNameArray.map((depName) => depName) +"]. " +
         "Page can't be hot-reloaded, firing full page reload.");
       window.location.reload();
     }
   } else {
     modulesGraph.set(name, []);
     for (const depName of depNameArray) {
       if (!\$dartLoader.moduleParentsGraph.has(depName)) {
         \$dartLoader.moduleParentsGraph.set(depName, []);
       }
       \$dartLoader.moduleParentsGraph.get(depName).push(name);
       modulesGraph.get(name).push(depName);
     }
   }
 };
 ''';

 /// Marker comment used by tools to identify the entrypoint file,
 /// to inject custom code.
 final _entrypointExtensionMarker = '/* ENTRYPOINT_EXTENTION_MARKER */';

 /// Marker comment used by tools to identify the main function
 /// to inject custom code.
 final _mainExtensionMarker = '/* MAIN_EXTENSION_MARKER */';

 final _baseUrlScript = '''
 var baseUrl = (function () {
   // Attempt to detect --precompiled mode for tests, and set the base url
   // appropriately, otherwise set it to '/'.
   var pathParts = location.pathname.split("/");
   if (pathParts[0] == "") {
     pathParts.shift();
   }
   if (pathParts.length > 1 && pathParts[1] == "test") {
     return "/" + pathParts.slice(0, 2).join("/") + "/";
   }
   // Attempt to detect base url using <base href> html tag
   // base href should start and end with "/"
   if (typeof document !== 'undefined') {
     var el = document.getElementsByTagName('base');
     if (el && el[0] && el[0].getAttribute("href") && el[0].getAttribute
     ("href").startsWith("/") && el[0].getAttribute("href").endsWith("/")){
       return el[0].getAttribute("href");
     }
   }
   // return default value
   return "/";
 }());
 ''';

 /// Ensures that all of [resources] are built successfully, and adds them as
 /// an input dependency to this action.
 ///
 /// This also has the effect of ensuring these resources are present whenever
 /// a DDC app is built - reducing the need to explicitly list these files as
 /// build filters.
 Future<void> _ensureResources(
     BuildStep buildStep, Iterable<AssetId> resources) async {
   for (var resource in resources) {
     if (!await buildStep.canRead(resource)) {
       throw StateError('Unable to locate required sdk resource $resource');
     }
   }
 }
	// Copyright (c) 2017, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.

	import 'dart:async';
	import 'dart:collection';
	import 'dart:convert';

	import 'package:build/build.dart';
	import 'package:build_modules/build_modules.dart';
	import 'package:meta/meta.dart';
	import 'package:path/path.dart' as _p; // ignore: library_prefixes
	import 'package:pool/pool.dart';

	import 'ddc_names.dart';
	import 'dev_compiler_builder.dart';
	import 'platforms.dart';
	import 'web_entrypoint_builder.dart';

	/// Alias `_p.url` to `p`.
	_p.Context get _context => _p.url;

	var _modulePartialExtension = _context.withoutExtension(jsModuleExtension);

	/// Bootstraps a ddc application, creating the main entrypoint as well as the
	/// bootstrap and digest entrypoints.
	///
	/// If [skipPlatformCheck] is `true` then all `dart:` imports will be
	/// allowed in all packages.
	///
	/// Deprecated: If [skipPlatformCheckPackages] is provided then any dart:
	/// imports will be allowed in the specified packages.
	///
	/// If [requiredAssets] is provided then this will ensure those assets are
	/// available to the app by making them inputs of this build action.
	Future<void> bootstrapDdc(
	BuildStep buildStep, {
	DartPlatform platform,
	bool skipPlatformCheck = false,
	@deprecated Set<String> skipPlatformCheckPackages = const {},
	Iterable<AssetId> requiredAssets,
	}) async {
	requiredAssets ??= [];
	skipPlatformCheck ??= false;
	// Ensures that the sdk resources are built and available.
	await _ensureResources(buildStep, requiredAssets);

	var dartEntrypointId = buildStep.inputId;
	var moduleId = buildStep.inputId
	.changeExtension(moduleExtension(platform ?? ddcPlatform));
	var module = Module.fromJson(json
	.decode(await buildStep.readAsString(moduleId)) as Map<String, dynamic>);

	// First, ensure all transitive modules are built.
	List<AssetId> transitiveJsModules;
	try {
	transitiveJsModules = await _ensureTransitiveJsModules(module, buildStep,
	skipPlatformCheck: skipPlatformCheck,
	skipPlatformCheckPackages: skipPlatformCheckPackages);
	} on UnsupportedModules catch (e) {
	var librariesString = (await e.exactLibraries(buildStep).toList())
	.map((lib) => AssetId(lib.id.package,
	lib.id.path.replaceFirst(moduleLibraryExtension, '.dart')))
	.join('\n');
	log.warning('''
	Skipping compiling ${buildStep.inputId} with ddc because some of its
	transitive libraries have sdk dependencies that not supported on this platform:

	$librariesString

	https://github.com/dart-lang/build/blob/master/docs/faq.md#how-can-i-resolve-skipped-compiling-warnings
	''');
	return;
	}
	var jsId = module.primarySource.changeExtension(jsModuleExtension);
	var appModuleName = ddcModuleName(jsId);
	var appDigestsOutput =
	dartEntrypointId.changeExtension(digestsEntrypointExtension);

	// The name of the entrypoint dart library within the entrypoint JS module.
	//
	// This is used to invoke `main()` from within the bootstrap script.
	//
	// TODO(jakemac53): Sane module name creation, this only works in the most
	// basic of cases.
	//
	// See https://github.com/dart-lang/sdk/issues/27262 for the root issue
	// which will allow us to not rely on the naming schemes that dartdevc uses
	// internally, but instead specify our own.
	var oldAppModuleScope = toJSIdentifier(
	_context.withoutExtension(_context.basename(buildStep.inputId.path)));

	// Like above but with a package-relative entrypoint.
	var appModuleScope =
	pathToJSIdentifier(_context.withoutExtension(buildStep.inputId.path));

	// Map from module name to module path for custom modules.
	var modulePaths = SplayTreeMap.of(
	{'dart_sdk': r'packages/build_web_compilers/src/dev_compiler/dart_sdk'});
	for (var jsId in transitiveJsModules) {
	// Strip out the top level dir from the path for any module, and set it to
	// `packages/` for lib modules. We set baseUrl to `/` to simplify things,
	// and we only allow you to serve top level directories.
	var moduleName = ddcModuleName(jsId);
	modulePaths[moduleName] = _context.withoutExtension(
	jsId.path.startsWith('lib')
	? '$moduleName$jsModuleExtension'
	: _context.joinAll(_context.split(jsId.path).skip(1)));
	}

	var bootstrapId = dartEntrypointId.changeExtension(ddcBootstrapExtension);
	var bootstrapModuleName = _context.withoutExtension(_context.relative(
	bootstrapId.path,
	from: _context.dirname(dartEntrypointId.path)));

	var dartEntrypointParts = _context.split(dartEntrypointId.path);
	var entrypointLibraryName = _context.joinAll([
	// Convert to a package: uri for files under lib.
	if (dartEntrypointParts.first == 'lib')
	'package:${module.primarySource.package}',
	// Strip top-level directory from the path.
	...dartEntrypointParts.skip(1),
	]);

	var bootstrapContent =
	StringBuffer('$_entrypointExtensionMarker\n(function() {\n')
	..write(_dartLoaderSetup(
	modulePaths,
	_p.url.relative(appDigestsOutput.path,
	from: _p.url.dirname(bootstrapId.path))))
	..write(_requireJsConfig)
	..write(_appBootstrap(bootstrapModuleName, appModuleName,
	appModuleScope, entrypointLibraryName,
	oldModuleScope: oldAppModuleScope));

	await buildStep.writeAsString(bootstrapId, bootstrapContent.toString());

	var entrypointJsContent = _entryPointJs(bootstrapModuleName);
	await buildStep.writeAsString(
	dartEntrypointId.changeExtension(jsEntrypointExtension),
	entrypointJsContent);

	// Output the digests for transitive modules.
	// These can be consumed for hot reloads.
	var moduleDigests = <String, String>{
	for (var jsId in transitiveJsModules)
	_moduleDigestKey(jsId): '${await buildStep.digest(jsId)}',
	};
	await buildStep.writeAsString(appDigestsOutput, jsonEncode(moduleDigests));
	}

	String _moduleDigestKey(AssetId jsId) =>
	'${ddcModuleName(jsId)}$jsModuleExtension';

	final _lazyBuildPool = Pool(16);

	/// Ensures that all transitive js modules for [module] are available and built.
	///
	/// Throws an [UnsupportedModules] exception if there are any
	/// unsupported modules.
	Future<List<AssetId>> _ensureTransitiveJsModules(
	Module module, BuildStep buildStep,
	{@required bool skipPlatformCheck,
	@required Set<String> skipPlatformCheckPackages}) async {
	// Collect all the modules this module depends on, plus this module.
	var transitiveDeps = await module.computeTransitiveDependencies(buildStep,
	throwIfUnsupported: !skipPlatformCheck,
	// ignore: deprecated_member_use
	skipPlatformCheckPackages: skipPlatformCheckPackages);

	var jsModules = [
	module.primarySource.changeExtension(jsModuleExtension),
	for (var dep in transitiveDeps)
	dep.primarySource.changeExtension(jsModuleExtension),
	];
	// Check that each module is readable, and warn otherwise.
	await Future.wait(jsModules.map((jsId) async {
	if (await _lazyBuildPool.withResource(() => buildStep.canRead(jsId))) {
	return;
	}
	var errorsId = jsId.addExtension('.errors');
	await buildStep.canRead(errorsId);
	log.warning('Unable to read $jsId, check your console or the '
	'`.dart_tool/build/generated/${errorsId.package}/${errorsId.path}` '
	'log file.');
	}));
	return jsModules;
	}

	/// Code that actually imports the [moduleName] module, and calls the
	/// `[moduleScope].main()` function on it.
	///
	/// Also performs other necessary initialization.
	String _appBootstrap(String bootstrapModuleName, String moduleName,
	String moduleScope, String entrypointLibraryName,
	{String oldModuleScope}) =>
	'''
	define("$bootstrapModuleName", ["$moduleName", "dart_sdk"], function(app, dart_sdk) {
	dart_sdk.dart.setStartAsyncSynchronously(true);
	dart_sdk._isolate_helper.startRootIsolate(() => {}, []);
	$_initializeTools
	$_mainExtensionMarker
	(app.$moduleScope \|\| app.$oldModuleScope).main();
	var bootstrap = {
	hot\$onChildUpdate: function(childName, child) {
	// Special handling for the multi-root scheme uris. We need to strip
	// out the scheme and the top level directory, to match the source path
	// that chrome sees.
	if (childName.startsWith('$multiRootScheme:///')) {
	childName = childName.substring('$multiRootScheme:///'.length);
	var firstSlash = childName.indexOf('/');
	if (firstSlash == -1) return false;
	childName = childName.substring(firstSlash + 1);
	}
	if (childName === "$entrypointLibraryName") {
	// Clear static caches.
	dart_sdk.dart.hotRestart();
	child.main();
	return true;
	}
	}
	}
	dart_sdk.dart.trackLibraries("$bootstrapModuleName", {
	"$bootstrapModuleName": bootstrap
	}, '');
	return {
	bootstrap: bootstrap
	};
	});
	})();
	''';

	/// The actual entrypoint JS file which injects all the necessary scripts to
	/// run the app.
	String _entryPointJs(String bootstrapModuleName) => '''
	(function() {
	$_currentDirectoryScript
	$_baseUrlScript

	var mapperUri = baseUrl + "packages/build_web_compilers/src/" +
	"dev_compiler_stack_trace/stack_trace_mapper.dart.js";
	var requireUri = baseUrl +
	"packages/build_web_compilers/src/dev_compiler/require.js";
	var mainUri = _currentDirectory + "$bootstrapModuleName";

	if (typeof document != 'undefined') {
	var el = document.createElement("script");
	el.defer = true;
	el.async = false;
	el.src = mapperUri;
	document.head.appendChild(el);

	el = document.createElement("script");
	el.defer = true;
	el.async = false;
	el.src = requireUri;
	el.setAttribute("data-main", mainUri);
	document.head.appendChild(el);
	} else {
	importScripts(mapperUri, requireUri);
	require.config({
	baseUrl: baseUrl,
	});
	// TODO: update bootstrap code to take argument - dart-lang/build#1115
	window = self;
	require([mainUri + '.js']);
	}
	})();
	''';

	/// JavaScript snippet to determine the directory a script was run from.
	final _currentDirectoryScript = r'''
	var _currentDirectory = (function () {
	var _url;
	var lines = new Error().stack.split('\n');
	function lookupUrl() {
	if (lines.length > 2) {
	var match = lines[1].match(/^\s+at (.+):\d+:\d+$/);
	// Chrome.
	if (match) return match[1];
	// Chrome nested eval case.
	match = lines[1].match(/^\s+at eval [(](.+):\d+:\d+[)]$/);
	if (match) return match[1];
	// Edge.
	match = lines[1].match(/^\s+at.+\((.+):\d+:\d+\)$/);
	if (match) return match[1];
	// Firefox.
	match = lines[0].match(/[<][@](.+):\d+:\d+$/)
	if (match) return match[1];
	}
	// Safari.
	return lines[0].match(/(.+):\d+:\d+$/)[1];
	}
	_url = lookupUrl();
	var lastSlash = _url.lastIndexOf('/');
	if (lastSlash == -1) return _url;
	var currentDirectory = _url.substring(0, lastSlash + 1);
	return currentDirectory;
	})();
	''';

	/// Sets up `window.$dartLoader` based on [modulePaths].
	String _dartLoaderSetup(Map<String, String> modulePaths, String appDigests) =>
	'''
	$_currentDirectoryScript
	$_baseUrlScript
	let modulePaths = ${const JsonEncoder.withIndent(" ").convert(modulePaths)};
	if(!window.\$dartLoader) {
	window.\$dartLoader = {
	appDigests: _currentDirectory + '$appDigests',
	moduleIdToUrl: new Map(),
	urlToModuleId: new Map(),
	rootDirectories: new Array(),
	// Used in package:build_runner/src/server/build_updates_client/hot_reload_client.dart
	moduleParentsGraph: new Map(),
	moduleLoadingErrorCallbacks: new Map(),
	forceLoadModule: function (moduleName, callback, onError) {
	// dartdevc only strips the final extension when adding modules to source
	// maps, so we need to do the same.
	if (moduleName.endsWith('$_modulePartialExtension')) {
	moduleName = moduleName.substring(0, moduleName.length - ${_modulePartialExtension.length});
	}
	if (typeof onError != 'undefined') {
	var errorCallbacks = \$dartLoader.moduleLoadingErrorCallbacks;
	if (!errorCallbacks.has(moduleName)) {
	errorCallbacks.set(moduleName, new Set());
	}
	errorCallbacks.get(moduleName).add(onError);
	}
	requirejs.undef(moduleName);
	requirejs([moduleName], function() {
	if (typeof onError != 'undefined') {
	errorCallbacks.get(moduleName).delete(onError);
	}
	if (typeof callback != 'undefined') {
	callback();
	}
	});
	},
	getModuleLibraries: null, // set up by _initializeTools
	};
	}
	let customModulePaths = {};
	window.\$dartLoader.rootDirectories.push(window.location.origin + baseUrl);
	for (let moduleName of Object.getOwnPropertyNames(modulePaths)) {
	let modulePath = modulePaths[moduleName];
	if (modulePath != moduleName) {
	customModulePaths[moduleName] = modulePath;
	}
	var src = window.location.origin + '/' + modulePath + '.js';
	if (window.\$dartLoader.moduleIdToUrl.has(moduleName)) {
	continue;
	}
	\$dartLoader.moduleIdToUrl.set(moduleName, src);
	\$dartLoader.urlToModuleId.set(src, moduleName);
	}
	''';

	/// Code to initialize the dev tools formatter, stack trace mapper, and any
	/// other tools.
	///
	/// Posts a message to the window when done.
	final _initializeTools = '''
	$_baseUrlScript
	dart_sdk._debugger.registerDevtoolsFormatter();
	\$dartLoader.getModuleLibraries = dart_sdk.dart.getModuleLibraries;
	if (window.\$dartStackTraceUtility && !window.\$dartStackTraceUtility.ready) {
	window.\$dartStackTraceUtility.ready = true;
	let dart = dart_sdk.dart;
	window.\$dartStackTraceUtility.setSourceMapProvider(
	function(url) {
	url = url.replace(baseUrl, '/');
	var module = window.\$dartLoader.urlToModuleId.get(url);
	if (!module) return null;
	return dart.getSourceMap(module);
	});
	}
	if (typeof document != 'undefined') {
	window.postMessage({ type: "DDC_STATE_CHANGE", state: "start" }, "*");
	}
	''';

	/// Require JS config for ddc.
	///
	/// Sets the base url to `/` so that all modules can be loaded using absolute
	/// paths which simplifies a lot of scenarios.
	///
	/// Sets the timeout for loading modules to infinity (0).
	///
	/// Sets up the custom module paths.
	///
	/// Adds error handler code for require.js which requests a `.errors` file for
	/// any failed module, and logs it to the console.
	final _requireJsConfig = '''
	// Whenever we fail to load a JS module, try to request the corresponding
	// `.errors` file, and log it to the console.
	(function() {
	var oldOnError = requirejs.onError;
	requirejs.onError = function(e) {
	if (e.requireModules) {
	if (e.message) {
	// If error occurred on loading dependencies, we need to invalidate ancessor too.
	var ancesor = e.message.match(/needed by: (.*)/);
	if (ancesor) {
	e.requireModules.push(ancesor[1]);
	}
	}
	for (const module of e.requireModules) {
	var errorCallbacks = \$dartLoader.moduleLoadingErrorCallbacks.get(module);
	if (errorCallbacks) {
	for (const callback of errorCallbacks) callback(e);
	errorCallbacks.clear();
	}
	}
	}
	if (e.originalError && e.originalError.srcElement) {
	var xhr = new XMLHttpRequest();
	xhr.onreadystatechange = function() {
	if (this.readyState == 4) {
	var message;
	if (this.status == 200) {
	message = this.responseText;
	} else {
	message = "Unknown error loading " + e.originalError.srcElement.src;
	}
	console.error(message);
	var errorEvent = new CustomEvent(
	'dartLoadException', { detail: message });
	window.dispatchEvent(errorEvent);
	}
	};
	xhr.open("GET", e.originalError.srcElement.src + ".errors", true);
	xhr.send();
	}
	// Also handle errors the normal way.
	if (oldOnError) oldOnError(e);
	};
	}());

	$_baseUrlScript;

	require.config({
	baseUrl: baseUrl,
	waitSeconds: 0,
	paths: customModulePaths
	});

	const modulesGraph = new Map();
	function getRegisteredModuleName(moduleMap) {
	if (\$dartLoader.moduleIdToUrl.has(moduleMap.name + '$_modulePartialExtension')) {
	return moduleMap.name + '$_modulePartialExtension';
	}
	return moduleMap.name;
	}
	requirejs.onResourceLoad = function (context, map, depArray) {
	const name = getRegisteredModuleName(map);
	const depNameArray = depArray.map(getRegisteredModuleName);
	if (modulesGraph.has(name)) {
	// TODO Move this logic to better place
	var previousDeps = modulesGraph.get(name);
	var changed = previousDeps.length != depNameArray.length;
	changed = changed \|\| depNameArray.some(function(depName) {
	return !previousDeps.includes(depName);
	});
	if (changed) {
	console.warn("Dependencies graph change for module '" + name + "' detected. " +
	"Dependencies was [" + previousDeps + "], now [" + depNameArray.map((depName) => depName) +"]. " +
	"Page can't be hot-reloaded, firing full page reload.");
	window.location.reload();
	}
	} else {
	modulesGraph.set(name, []);
	for (const depName of depNameArray) {
	if (!\$dartLoader.moduleParentsGraph.has(depName)) {
	\$dartLoader.moduleParentsGraph.set(depName, []);
	}
	\$dartLoader.moduleParentsGraph.get(depName).push(name);
	modulesGraph.get(name).push(depName);
	}
	}
	};
	''';

	/// Marker comment used by tools to identify the entrypoint file,
	/// to inject custom code.
	final _entrypointExtensionMarker = '/* ENTRYPOINT_EXTENTION_MARKER */';

	/// Marker comment used by tools to identify the main function
	/// to inject custom code.
	final _mainExtensionMarker = '/* MAIN_EXTENSION_MARKER */';

	final _baseUrlScript = '''
	var baseUrl = (function () {
	// Attempt to detect --precompiled mode for tests, and set the base url
	// appropriately, otherwise set it to '/'.
	var pathParts = location.pathname.split("/");
	if (pathParts[0] == "") {
	pathParts.shift();
	}
	if (pathParts.length > 1 && pathParts[1] == "test") {
	return "/" + pathParts.slice(0, 2).join("/") + "/";
	}
	// Attempt to detect base url using <base href> html tag
	// base href should start and end with "/"
	if (typeof document !== 'undefined') {
	var el = document.getElementsByTagName('base');
	if (el && el[0] && el[0].getAttribute("href") && el[0].getAttribute
	("href").startsWith("/") && el[0].getAttribute("href").endsWith("/")){
	return el[0].getAttribute("href");
	}
	}
	// return default value
	return "/";
	}());
	''';

	/// Ensures that all of [resources] are built successfully, and adds them as
	/// an input dependency to this action.
	///
	/// This also has the effect of ensuring these resources are present whenever
	/// a DDC app is built - reducing the need to explicitly list these files as
	/// build filters.
	Future<void> _ensureResources(
	BuildStep buildStep, Iterable<AssetId> resources) async {
	for (var resource in resources) {
	if (!await buildStep.canRead(resource)) {
	throw StateError('Unable to locate required sdk resource $resource');
	}
	}
	}