Add null check.

tests/compiler/dart2js/unparser_test.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.
 
 #import('parser_helper.dart');
 #import("../../../lib/compiler/implementation/tree/tree.dart");
 
 testUnparse(String statement) {
   Node node = parseStatement(statement);
   Expect.equals(statement, node.unparse());
@@ skipping 13 matching lines @@
 
 testForLoop() {
   testUnparse('for(;i<100;i++){}');
   testUnparse('for(i=0;i<100;i++){}');
 }
 
 testEmptyList() {
   testUnparse('var x= [];');
 }
 
+testClosure() {
+  testUnparse('var x=(var x)=> x;');
+}
+
 testIndexedOperatorDecl() {
   testUnparseMember('operator[](int i)=> null;');
 }
 
 main() {
   testGenericTypes();
   testForLoop();
   testEmptyList();
+  testClosure();
   testIndexedOperatorDecl();
 }