fixes #221, initial sync*, async, async* implementation

lib/src/js/template.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.
 
 part of js_ast;
 
 class TemplateManager {
   Map<String, Template> expressionTemplates = new Map<String, Template>();
   Map<String, Template> statementTemplates = new Map<String, Template>();
 
@@ skipping 625 matching lines @@
   Instantiator visitThis(This node) => (arguments) => new This();
   Instantiator visitSuper(Super node) => (arguments) => new Super();
 
   Instantiator visitIdentifier(Identifier node) =>
       (arguments) => new Identifier(node.name);
 
   Instantiator visitSpread(Spread node) =>
       (args) => new Spread(visit(node.argument)(args));
 
   Instantiator visitYield(Yield node) =>
-      (args) => new Yield(visit(node.value)(args), star: node.star);
+      (args) => new Yield(
+          node.value != null ? visit(node.value)(args) : null, star: node.star);
 
   Instantiator visitRestParameter(RestParameter node) =>
       (args) => new RestParameter(visit(node.parameter)(args));
 
   Instantiator visitAccess(PropertyAccess node) {
     Instantiator makeReceiver = visit(node.receiver);
     Instantiator makeSelector = visit(node.selector);
     return (arguments) =>
         new PropertyAccess(makeReceiver(arguments), makeSelector(arguments));
   }
@@ skipping 192 matching lines @@
     if (count != before) containsInterpolatedNode.add(node);
     return null;
   }
 
   visitInterpolatedNode(InterpolatedNode node) {
     containsInterpolatedNode.add(node);
     if (node.isNamed) holeNames.add(node.nameOrPosition);
     ++count;
   }
 }