Make support for async/await syntax optional in the frontend.

pkg/compiler/lib/src/dart_backend/backend.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 dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
   final Node ast;
   final TreeElements treeElements;
 
   ElementAst(this.ast, this.treeElements);
 }
 
 class DartBackend extends Backend {
   final List<CompilerTask> tasks;
   final bool stripAsserts;
 
   bool get supportsReflection => true;
 
+  bool get supportsAsyncAwait => true;
+
   // TODO(zarah) Maybe change this to a command-line option.
   // Right now, it is set by the tests.
   bool useMirrorHelperLibrary = false;
 
   /// Updated to a [MirrorRenamerImpl] instance if the [useMirrorHelperLibrary]
   /// field is set and mirror are needed.
   MirrorRenamer mirrorRenamer = const MirrorRenamer();
 
   final DartOutputter outputter;
 
@@ skipping 498 matching lines @@
   }
 
   // TODO(johnniwinther): Remove this when values are computed from the
   // expressions.
   @override
   void copyConstantValues(DartConstantTask task) {
     constantCompiler.constantValueMap
         .addAll(task.constantCompiler.constantValueMap);
   }
 }