Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/size_test.dart

 // Copyright (c) 2013, 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 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import "compiler_helper.dart";
 
 const String TEST = "main() => [];";
 
@@ skipping 11 matching lines @@
   'Map': 'class Map {}',
   'StackTrace': 'class StackTrace {}',
   'identical': 'identical(a, b) => true;',
   'proxy': 'const proxy = 0;',
 };
 
 main() {
   asyncTest(() =>
       compileAll(TEST, coreSource: DEFAULT_CORELIB_WITH_LIST).then((generated) {
         return MockCompiler.create((MockCompiler compiler) {
-          var backend = compiler.backend;
-
           // Make sure no class is emitted.
           Expect.isFalse(generated.contains('finishClasses'));
         });
       }));
 }