Support async/await in dart2dart.

pkg/compiler/lib/src/resolution/members.dart

 // Copyright (c) 2012, 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 resolution;
 
 abstract class TreeElements {
   AnalyzableElement get analyzedElement;
   Iterable<Node> get superUses;
 
@@ skipping 482 matching lines @@
 
   static void processAsyncMarker(Compiler compiler,
                                  BaseFunctionElementX element) {
     FunctionExpression functionExpression = element.node;
     AsyncModifier asyncModifier = functionExpression.asyncModifier;
     if (asyncModifier != null) {
       if (!compiler.enableAsyncAwait) {
         compiler.reportError(asyncModifier,
             MessageKind.EXPERIMENTAL_ASYNC_AWAIT,
             {'modifier': element.asyncMarker});
-      } else if (!compiler.analyzeOnly) {
-        compiler.reportError(asyncModifier,
-            MessageKind.EXPERIMENTAL_ASYNC_AWAIT,
-            {'modifier': element.asyncMarker});
       }
 
       if (asyncModifier.isAsynchronous) {
         element.asyncMarker = asyncModifier.isYielding
             ? AsyncMarker.ASYNC_STAR : AsyncMarker.ASYNC;
       } else {
         element.asyncMarker = AsyncMarker.SYNC_STAR;
       }
       if (element.isAbstract) {
         compiler.reportError(asyncModifier,
@@ skipping 4513 matching lines @@
 }
 
 /// The result for the resolution of the `assert` method.
 class AssertResult implements ResolutionResult {
   const AssertResult();
 
   Element get element => null;
 
   String toString() => 'AssertResult()';
 }