fix #43, remove => workaround

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 1120 matching lines @@
   Fun _clone() => new Fun(params, body,
       isGenerator: isGenerator, asyncModifier: asyncModifier);
 
   int get precedenceLevel => PRIMARY_LOW_PRECEDENCE;
 }
 
 class ArrowFun extends FunctionExpression {
   final List<Parameter> params;
   final body; // Expression or Block
 
-  bool _closesOverThis; // lazy initialized
-
   ArrowFun(this.params, this.body);
 
   accept(NodeVisitor visitor) => visitor.visitArrowFun(this);
 
   void visitChildren(NodeVisitor visitor) {
     for (Parameter param in params) param.accept(visitor);
     body.accept(visitor);
   }
-  /// True if this function actually closes of `this`. We use this in some
-  /// situations to generate different code.
-  bool get closesOverThis {
-    if (_closesOverThis == null) _closesOverThis = This.foundIn(this);
-    return _closesOverThis;
-  }
 
   int get precedenceLevel => PRIMARY_LOW_PRECEDENCE;
 
   ArrowFun _clone() => new ArrowFun(params, body);
 }
 
 /**
  * The Dart sync, sync*, async, and async* modifier.
  * See [DartYield].
  *
@@ skipping 596 matching lines @@
 
   final List<ModuleItem> body;
   Module(this.body, {this.name});
 
   accept(NodeVisitor visitor) => visitor.visitModule(this);
   void visitChildren(NodeVisitor visitor) {
     for (ModuleItem item in body) item.accept(visitor);
   }
   Module _clone() => new Module(body);
 }