Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/dart2js_batch_test.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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:async_helper/async_helper.dart';
@@ skipping 68 matching lines @@
     Expect.equals(4, ">>> EOF STDERR".allMatches(stderrOutput).length);
     Expect.equals(4, ">>>".allMatches(stderrOutput).length);
 
     Expect.equals(2, ">>> TEST OK".allMatches(stdoutOutput).length);
     Expect.equals(2, ">>> TEST FAIL".allMatches(stdoutOutput).length);
     Expect.equals(4, ">>>".allMatches(stdoutOutput).length);
   });
 }
 
 void main() {
-  var tmpDir;
   asyncTest(() {
     return setup().then(launchDart2Js).then(runTests).whenComplete(cleanUp);
   });
 }