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

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

Index: pkg/front_end/lib/src/fasta/target.dart
diff --git a/pkg/front_end/lib/src/fasta/target.dart b/pkg/front_end/lib/src/fasta/target.dart
index 76f8ad3569a35e62f601ad0a03aa02013056b8a5..d66156b645df721c4862c48e9f146a8ce9a67abd 100644
--- a/pkg/front_end/lib/src/fasta/target.dart
+++ b/pkg/front_end/lib/src/fasta/target.dart
@@ -15,7 +15,18 @@ import 'ticker.dart' show Ticker;
 abstract class Target {
   final Ticker ticker;
 
-  Target(this.ticker);
+  /// Denotes whether the output of this target will be used for dart2js,
+  /// otherwise it is assumed it will be used for the VM.
+  // TODO(sigmund): investigate better ways to configure this. This is used for
+  // three purposes today: determine how to handle the `native` clauses (which
+  // are different in the vm and dartjs patch-files), determine which libraries
+  // to include when building the sdk, and which transformations to run when
+  // linking a program.
+  final bool forDart2js;
+
+  bool get forVm => !forDart2js;
+
+  Target(this.ticker, this.forDart2js);
 
   /// Instructs this target to include [uri] in its result.
   void read(Uri uri);