dart2js cps: Support async/await by rewriting the JavaScript AST.

pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart

 // Copyright (c) 2014, 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 dart2js.ir_tracer;
 
 import 'dart:async' show EventSink;
 import 'cps_ir_nodes.dart' as cps_ir hide Function;
 import '../tracer.dart';
 
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     String index = formatReference(node.index);
     return 'GetIndex $object $index';
   }
 
   visitSetIndex(cps_ir.SetIndex node) {
     String object = formatReference(node.object);
     String index = formatReference(node.index);
     String value = formatReference(node.value);
     return 'SetIndex $object $index $value';
   }
+
+  @override
+  visitAwait(cps_ir.Await node) {
+    String value = formatReference(node.input);
+    String continuation = formatReference(node.continuation);
+    return 'Await $value $continuation';
+  }
 }
 
 /**
  * Invents (and remembers) names for Continuations, Parameters, etc.
  * The names must match the conventions used by IR Hydra, e.g.
  * Continuations and Functions must have names of form B### since they
  * are visualized as basic blocks.
  */
 class Names {
   final Map<Object, String> names = {};
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   }
 
   visitSetStatic(cps_ir.SetStatic node) {
     unexpectedNode(node);
   }
 
   @override
   visitForeignCode(cps_ir.ForeignCode node) {
     addEdgeToContinuation(node.continuation);
   }
+
+  @override
+  visitAwait(cps_ir.Await node) {
+    unexpectedNode(node);
+  }
 }