Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/js_spec_optimization_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:expect/expect.dart';
 import 'compiler_helper.dart';
 
 const String TEST_1 = r"""
   import 'dart:_foreign_helper';
   main() {
     // present: 'Moose'
     JS('', 'Moose');
 
     // absent: 'Phantom' - pure.
     JS('returns: bool;effects:none;depends:none;throws:never', 'Phantom');
@@ skipping 80 matching lines @@
   }
 
   asyncTest(() => Future.wait([
         check(TEST_1),
         check(TEST_2),
         check(TEST_3),
         check(TEST_4),
         check(TEST_5),
       ]));
 }