Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/scanner/partial_parser.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;
 
 class PartialParser extends Parser {
   PartialParser(Listener listener) : super(listener);
 
   Token parseClassBody(Token token) => skipClassBody(token);
@@ skipping 85 matching lines @@
     BeginGroupToken beginGroupToken = token;
     Token endGroup = beginGroupToken.endGroup;
     if (endGroup == null) {
       return listener.unmatched(beginGroupToken);
     } else if (!identical(endGroup.kind, $CLOSE_CURLY_BRACKET)) {
       return listener.unmatched(beginGroupToken);
     }
     return endGroup;
   }
 
+  Token skipAsyncModifier(Token token) {
+    String value = token.stringValue;
+    if (identical(value, 'async')) {
+      token = token.next;
+      value = token.stringValue;
+
+      if (identical(value, '*')) {
+        token = token.next;
+      }
+    } else if (identical(value, 'sync')) {
+      token = token.next;
+      value = token.stringValue;
+
+      if (identical(value, '*')) {
+        token = token.next;
+      }
+    }
+    return token;
+  }
+
   Token parseFunctionBody(Token token, bool isExpression, bool allowAbstract) {
     assert(!isExpression);
+    token = skipAsyncModifier(token);
     String value = token.stringValue;
     if (identical(value, ';')) {
       if (!allowAbstract) {
         listener.reportError(token, MessageKind.BODY_EXPECTED);
       }
       listener.handleNoFunctionBody(token);
     } else {
       if (identical(value, '=>')) {
         token = parseExpression(token.next);
         expectSemicolon(token);
@@ skipping 18 matching lines @@
         return token;
       }
       return listener.unexpected(token);
     }
     BeginGroupToken beginGroupToken = token;
     Token endToken = beginGroupToken.endGroup;
     listener.endFormalParameters(0, token, endToken);
     return endToken.next;
   }
 }