Implementation of async-await transformation on js ast.

pkg/compiler/lib/src/ssa/ssa_tracer.dart

 // Copyright (c) 2013, 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.
 
 library ssa.tracer;
 
 import 'dart:async' show EventSink;
 
 import 'ssa.dart';
 import '../js_backend/js_backend.dart';
@@ skipping 501 matching lines @@
     return "VoidType";
   }
 
   String visitInterfaceType(HInterfaceType node) {
     return "InterfaceType: ${node.dartType}";
   }
 
   String visitDynamicType(HDynamicType node) {
     return "DynamicType";
   }
+
+  String visitAwait(HAwait node) {
+    return "await ${temporaryId(node.inputs[0])}";
+  }
+
+  String visitYield(HYield node) {
+    return "yield${node.hasStar ? "*" : ""} ${temporaryId(node.inputs[0])}";
+  }
 }