[build] Prevent the Flutter asset packaging tool from making network calls.

These calls would stall for an hour in virual environments.

Bug: 48828
Change-Id: Iaca7125bd263bed356f2b1e3807a6b3efdbeb403
diff --git a/runtime/flutter_runner/build/asset_package.py b/runtime/flutter_runner/build/asset_package.py
index c868887..1d5235f 100755
--- a/runtime/flutter_runner/build/asset_package.py
+++ b/runtime/flutter_runner/build/asset_package.py
@@ -31,6 +31,8 @@
 
   env = os.environ.copy()
   env['FLUTTER_ROOT'] = args.flutter_root
+  # This ensures the tool does not make unneeded network calls.
+  env['BOT'] = 'true'
 
   call_args = [
     args.flutter_tools,