Add support for building patched_sdk and platform.dill for dart2js:

pkg/front_end/lib/src/fasta/compile_platform.dart

Index: pkg/front_end/lib/src/fasta/compile_platform.dart
diff --git a/pkg/front_end/lib/src/fasta/compile_platform.dart b/pkg/front_end/lib/src/fasta/compile_platform.dart
index 9be271f72e20219d286bf9409599018cc0da22db..b85f4d7da250d6b0a10dcea108912db332c12631 100644
--- a/pkg/front_end/lib/src/fasta/compile_platform.dart
+++ b/pkg/front_end/lib/src/fasta/compile_platform.dart
@@ -65,9 +65,10 @@ Future compilePlatformInternal(
       await TranslateUri.parse(patchedSdk, c.options.packages);
   ticker.logMs("Read packages file");
 
-  DillTarget dillTarget = new DillTarget(ticker, uriTranslator);
-  KernelTarget kernelTarget = new KernelTarget(
-      dillTarget, uriTranslator, c.options.strongMode, c.uriToSource);
+  DillTarget dillTarget =
+      new DillTarget(ticker, uriTranslator, c.options.targetDart2js);

[ahe, 2017/04/27 13:34:34]

If DillTarget needs to know about dart2js, we should probably also make a
subclass for that.

[Siggi Cherem (dart-lang), 2017/04/28 21:37:20]

Done.

+  KernelTarget kernelTarget = new KernelTarget(dillTarget, uriTranslator,
+      c.options.strongMode, c.options.targetDart2js, c.uriToSource);
 
   kernelTarget.read(Uri.parse("dart:core"));
   await dillTarget.writeOutline(null);