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

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

Index: pkg/front_end/lib/src/fasta/target_implementation.dart
diff --git a/pkg/front_end/lib/src/fasta/target_implementation.dart b/pkg/front_end/lib/src/fasta/target_implementation.dart
index d702e6e7473b41879e437a776ef016c0f0500046..3b657a39140d308974775e2134c95a0f370d0d14 100644
--- a/pkg/front_end/lib/src/fasta/target_implementation.dart
+++ b/pkg/front_end/lib/src/fasta/target_implementation.dart
@@ -4,8 +4,12 @@
 
 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;
@@ -23,7 +27,8 @@ abstract class TargetImplementation extends Target {
   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.
@@ -73,7 +78,9 @@ abstract class TargetImplementation extends Target {
   }
 
   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));
     }
   }