Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/scanner/keyword.dart

 // Copyright (c) 2011, 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.
 
 part of scanner;
 
 /**
  * A keyword in the Dart programming language.
  */
 class Keyword {
@@ skipping 49 matching lines @@
       const Keyword("set", isBuiltIn: true),
       const Keyword("static", isBuiltIn: true),
       const Keyword("typedef", isBuiltIn: true),
 
       const Keyword("hide", isPseudo: true),
       const Keyword("native", isPseudo: true),
       const Keyword("of", isPseudo: true),
       const Keyword("on", isPseudo: true),
       const Keyword("show", isPseudo: true),
       const Keyword("source", isPseudo: true),
-      const Keyword("deferred", isPseudo: true)];
+      const Keyword("deferred", isPseudo: true),
+      const Keyword("async", isPseudo: true),
+      const Keyword("sync", isPseudo: true),
+      const Keyword("await", isPseudo: true),
+      const Keyword("yield", isPseudo: true)];
 
   final String syntax;
   final bool isPseudo;
   final bool isBuiltIn;
   final PrecedenceInfo info;
 
   static Map<String, Keyword> _keywords;
   static Map<String, Keyword> get keywords {
     if (_keywords == null) {
       _keywords = computeKeywordMap();
@@ skipping 115 matching lines @@
 /**
  * A state that has no outgoing transitions.
  */
 class LeafKeywordState extends KeywordState {
   LeafKeywordState(String syntax) : super(Keyword.keywords[syntax]);
 
   KeywordState next(int c) => null;
 
   String toString() => keyword.syntax;
 }