implement null aware ops, fixes #249

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 427 matching lines @@
 
   Statement toReturn() => this;
 
   accept(NodeVisitor visitor) => visitor.visitReturn(this);
 
   void visitChildren(NodeVisitor visitor) {
     if (value != null) value.accept(visitor);
   }
 
   Return _clone() => new Return(value);
+
+  static bool foundIn(Node node) {
+    _returnFinder.found = false;
+    node.accept(_returnFinder);
+    return _returnFinder.found;
+  }
 }
 
+final _returnFinder = new _ReturnFinder();
+class _ReturnFinder extends BaseVisitor {
+  bool found = false;
+  visitReturn(Return node) {
+    found = true;
+  }
+  visitNode(Node node) {
+    if (!found) super.visitNode(node);
+  }
+}
+
+
 class Throw extends Statement {
   final Expression expression;
 
   Throw(this.expression);
 
   accept(NodeVisitor visitor) => visitor.visitThrow(this);
 
   void visitChildren(NodeVisitor visitor) {
     expression.accept(visitor);
   }
@@ skipping 1074 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);
 }