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

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

 // Copyright (c) 2016, 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.
 
 library fasta.compile_platform;
 
 import 'dart:async' show Future;
 
 import 'ticker.dart' show Ticker;
 
@@ skipping 47 matching lines @@
   Uri deps = Uri.base.resolveUri(new Uri.file("${output.toFilePath()}.d"));
   ticker.logMs("Parsed arguments");
   if (ticker.isVerbose) {
     print("Compiling $patchedSdk to $output");
   }
 
   TranslateUri uriTranslator =
       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);
   await kernelTarget.writeOutline(output);
 
   if (exitCode != 0) return null;
   await kernelTarget.writeProgram(output,
       dumpIr: c.options.dumpIr, verify: c.options.verify);
   await kernelTarget.writeDepsFile(output, deps);
 }