[dart] Define dart.vm.product=true

Flutter uses this define to switch to 'release' mode and omit certain
debugging code, see
https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/foundation/constants.dart

Change-Id: I66b8eb65cb219f34609cdb7bbbf874b0f25b0583
diff --git a/runtime/dart/dart_kernel.gni b/runtime/dart/dart_kernel.gni
index 5c2f1f4..963b50e 100644
--- a/runtime/dart/dart_kernel.gni
+++ b/runtime/dart/dart_kernel.gni
@@ -149,7 +149,7 @@
     if (is_debug) {
       args += [ "--embed-sources" ]
     } else {
-      args += [ "--no-embed-sources" ]
+      args += [ "--no-embed-sources", "-Ddart.vm.product=true" ]
     }
 
     if (defined(invoker.gen_bytecode) && invoker.gen_bytecode) {