Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/use_unused_api.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.
 
 /// This file use methods that aren't used by dart2js.dart, but that we wish to
 /// keep anyway. This might be general API that isn't currently in use,
 /// debugging aids, or API only used for testing (see TODO below).
 
 library dart2js.use_unused_api;
 
@@ skipping 54 matching lines @@
 }
 
 void useConstant(constants.ConstantValue constant,
                  dart2jslib.ConstantSystem cs) {
   constant.isObject;
   cs.isBool(constant);
 }
 
 void useNode(tree.Node node) {
   node
+    ..asAsyncModifier()
+    ..asAwait()
     ..asBreakStatement()
     ..asCascade()
     ..asCatchBlock()
     ..asClassNode()
     ..asCombinator()
     ..asConditional()
     ..asContinueStatement()
     ..asErrorExpression()
     ..asExport()
     ..asFor()
@@ skipping 18 matching lines @@
     ..asStringInterpolation()
     ..asStringInterpolationPart()
     ..asStringJuxtaposition()
     ..asStringNode()
     ..asSwitchCase()
     ..asSwitchStatement()
     ..asTryStatement()
     ..asTypeAnnotation()
     ..asTypeVariable()
     ..asTypedef()
-    ..asWhile();
+    ..asWhile()
+    ..asYield();
 }
 
 void useUtil(util.Link link) {
   link.reversePrependAll(link);
   util.longestCommonPrefixLength(null, null);
   new util.Pair(null, null);
 }
 
 void useSetlet(util.Setlet setlet) {
   setlet.difference(setlet);
@@ skipping 121 matching lines @@
   compiler.forgetElement(null);
 }
 
 useTypes() {
   new dart_types.ResolvedTypedefType(null, null, null).unalias(null);
 }
 
 useCodeEmitterTask(js_emitter.CodeEmitterTask codeEmitterTask) {
   codeEmitterTask.oldEmitter.clearCspPrecompiledNodes();
 }