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

pkg/front_end/lib/src/fasta/target_implementation.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.target_implementation;
 
+import 'package:kernel/target/targets.dart' as kernel show Target;
+
 import 'package:kernel/target/vm.dart' show VmTarget;
 
+import 'package:kernel/target/dart2js.dart' show Dart2jsTarget;
+
 import 'builder/builder.dart' show Builder, ClassBuilder, LibraryBuilder;
 
 import 'loader.dart' show Loader;
 
 import 'target.dart' show Target;
 
 import 'ticker.dart' show Ticker;
 
 import 'translate_uri.dart' show TranslateUri;
 
 /// Provides the implementation details used by a loader for a target.
 abstract class TargetImplementation extends Target {
   final TranslateUri uriTranslator;
   Builder cachedCompileTimeError;
   Builder cachedAbstractClassInstantiationError;
   Builder cachedNativeAnnotation;
 
-  TargetImplementation(Ticker ticker, this.uriTranslator) : super(ticker);
+  TargetImplementation(Ticker ticker, this.uriTranslator, bool forDart2js)
+      : super(ticker, forDart2js);
 
   /// Creates a [LibraryBuilder] corresponding to [uri], if one doesn't exist
   /// already.
   LibraryBuilder createLibraryBuilder(Uri uri, Uri fileUri);
 
   /// Add the classes extended or implemented directly by [cls] to [set].
   void addDirectSupertype(ClassBuilder cls, Set<ClassBuilder> set);
 
   /// Returns all classes that will be included in the resulting program.
   List<ClassBuilder> collectAllClasses();
@@ skipping 29 matching lines @@
   /// Returns a reference to the constructor used for creating `native`
   /// annotations. The constructor is expected to accept a single argument of
   /// type String, which is the name of the native method.
   Builder getNativeAnnotation(Loader loader) {
     if (cachedNativeAnnotation != null) return cachedNativeAnnotation;
     LibraryBuilder internal = loader.read(Uri.parse("dart:_internal"));
     return cachedNativeAnnotation = internal.getConstructor("ExternalName");
   }
 
   void loadExtraRequiredLibraries(Loader loader) {
-    for (String uri in new VmTarget(null).extraRequiredLibraries) {
+    kernel.Target target =
+        forDart2js ? new Dart2jsTarget(null) : new VmTarget(null);
+    for (String uri in target.extraRequiredLibraries) {
       loader.read(Uri.parse(uri));
     }
   }
 
   void addSourceInformation(
       Uri uri, List<int> lineStarts, List<int> sourceCode);
 }