Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/no_such_method_enabled_test.dart

 // Copyright (c) 2015, 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 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/common_elements.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/entities.dart';
 import 'package:compiler/src/js_backend/no_such_method_registry.dart';
 import 'package:compiler/src/world.dart';
 import 'package:expect/expect.dart';
 import 'kernel/compiler_helper.dart';
 import 'compiler_helper.dart';
 
@@ skipping 330 matching lines @@
           registry.complexReturningImpls.contains(noSuchMethod),
           "Unexpected isComplexReturn result on $noSuchMethod.");
     }
   }
 
   Expect.equals(
       test.isNoSuchMethodUsed,
       closedWorld.backendUsage.isNoSuchMethodUsed,
       "Unexpected isNoSuchMethodUsed result.");
 }