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

lib/src/js/nodes.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 js_ast;
 
 abstract class NodeVisitor<T> {
   T visitProgram(Program node);
 
   T visitBlock(Block node);
@@ skipping 1241 matching lines @@
     template.accept(visitor);
   }
 
   TaggedTemplate _clone() => new TaggedTemplate(tag, template);
 
   int get precedenceLevel => CALL;
 }
 
 // TODO(jmesserly): parser does not support this yet.
 class Yield extends Expression {
-  final Expression value;
+  final Expression value; // Can be null.
 
   /**
    * Whether this yield expression is a `yield*` that iterates each item in
    * [value].
    */
   final bool star;
 
   Yield(this.value, {this.star: false});
 
   accept(NodeVisitor visitor) => visitor.visitYield(this);
 
   void visitChildren(NodeVisitor visitor) {
-    value.accept(visitor);
+    if (value != null) value.accept(visitor);
   }
 
   Yield _clone() => new Yield(value);
 
   int get precedenceLevel => YIELD;
 }
 
 class ClassDeclaration extends Statement {
   final ClassExpression classExpr;
 
@@ skipping 220 matching lines @@
   final Expression expression;
 
   CommentExpression(this.comment, this.expression);
 
   int get precedenceLevel => PRIMARY;
   accept(NodeVisitor visitor) => visitor.visitCommentExpression(this);
   CommentExpression _clone() => new CommentExpression(comment, expression);
 
   void visitChildren(NodeVisitor visitor) => expression.accept(visitor);
 }