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

pkg/front_end/lib/src/fasta/target.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;
 
 import 'dart:async' show Future;
 
 import 'ticker.dart' show Ticker;
 
 /// A compilation target.
 ///
 /// A target reads source files with [read] and writes out the resulting
 /// program when [writeOutline] is called.
 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);
 
   /// Write the resulting program in the file [uri].
   Future writeProgram(Uri uri);
 
   /// Write the resulting outline in the file [uri].
   Future writeOutline(Uri uri);
 }