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

pkg/front_end/lib/src/fasta/source/directive_listener.dart

Index: pkg/front_end/lib/src/fasta/source/directive_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/directive_listener.dart b/pkg/front_end/lib/src/fasta/source/directive_listener.dart
index 278b44a02bb6977e887a4ca222d5a215591b163c..355e3c75115fc70c3981213689bfa8ab24ade50e 100644
--- a/pkg/front_end/lib/src/fasta/source/directive_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/directive_listener.dart
@@ -7,7 +7,8 @@
 library front_end.src.fasta.source.directive_listener;
 
 import '../fasta_codes.dart' show FastaMessage, codeExpectedBlockToSkip;
-import '../parser/dart_vm_native.dart' show skipNativeClause;
+import '../parser/dart2js_native.dart' show skipDart2jsNativeClause;
+import '../parser/dart_vm_native.dart' show skipVmNativeClause;
 import '../parser/listener.dart';
 import '../quote.dart';
 import '../scanner/token.dart';
@@ -20,9 +21,13 @@ import '../scanner/token.dart';
 /// any top-level declaration, but we recommend to continue parsing the entire
 /// file in order to gracefully handle input errors.
 class DirectiveListener extends Listener {
-  /// Whether we accept the native-syntax used by the VM patch files.
+  /// Whether we accept the native-syntax used by patch files.
   final bool acceptsNativeClause;
 
+  /// In the precense of native clauses, whether they belong to dart2js patch
+  /// files (otherwise to VM patch files).
+  final bool targetingDart2js;
+
   /// Collects URIs that occur on any import directive.
   final Set<String> imports = new Set<String>();
 
@@ -32,7 +37,8 @@ class DirectiveListener extends Listener {
   /// Collects URIs that occur on any part directive.
   final Set<String> parts = new Set<String>();
 
-  DirectiveListener({this.acceptsNativeClause: false});
+  DirectiveListener(
+      {this.acceptsNativeClause: false, this.targetingDart2js: false});
 
   /// Set when entering the context of a directive, null when the parser is not
   /// looking at a directive.
@@ -80,7 +86,9 @@ class DirectiveListener extends Listener {
   @override
   Token handleUnrecoverableError(Token token, FastaMessage message) {
     if (acceptsNativeClause && message.code == codeExpectedBlockToSkip) {
-      Token recover = skipNativeClause(token);
+      Token recover = targetingDart2js
+          ? skipDart2jsNativeClause(token)
+          : skipVmNativeClause(token);
       if (recover != null) return recover;
     }
     return super.handleUnrecoverableError(token, message);