Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/tdiv_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.
 
 library tdiv_test;
 
 import 'dart:async';
-import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String TEST1 = r"""
 foo(param) {
   var a = param ? 0xFFFFFFFF : 1;
   return a ~/ 2;
   // Above can be compiled to division followed by truncate.
   // present: ' / 2 | 0'
   // absent: 'tdiv'
@@ skipping 48 matching lines @@
   }
 
   asyncTest(() => Future.wait([
         check(TEST1),
         check(TEST2),
         check(TEST3),
         check(TEST4),
         check(TEST5),
       ]));
 }