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

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.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 tree_ir.optimization.statement_rewriter;
 
 import 'optimization.dart' show Pass;
 import '../tree_ir_nodes.dart';
 import '../../io/source_information.dart';
 
@@ skipping 1089 matching lines @@
   Expression visitForeignExpression(ForeignExpression node) {
     _rewriteList(node.arguments);
     return node;
   }
 
   @override
   Statement visitForeignStatement(ForeignStatement node) {
     _rewriteList(node.arguments);
     return node;
   }
+
+  @override
+  Expression visitAwait(Await node) {
+    return node;
+  }
 }
 
 /// Result of combining two expressions, with the potential for reverting the
 /// combination.
 ///
 /// Reverting a combination is done by calling [uncombine]. In this case,
 /// both the original expressions should remain in the tree, and the [combined]
 /// expression should be orphaned.
 ///
 /// Explicitly reverting a combination is necessary to maintain variable
@@ skipping 77 matching lines @@
   VariableUseVisitor(this.callback);
 
   visitVariableUse(VariableUse use) => callback(use);
 
   visitInnerFunction(FunctionDefinition node) {}
 
   static void visit(Expression node, VariableUseCallback callback) {
     new VariableUseVisitor(callback).visitExpression(node);
   }
 }