Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/modulo_remainder_test.dart

 // Copyright (c) 2016, 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:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String MOD1 = r"""
 foo(param) {
   var a = param ? 0xFFFFFFFF : 1;
   return a % 2;
   // present: ' % 2'
   // absent: '$mod'
 }
@@ skipping 56 matching lines @@
 
   asyncTest(() => Future.wait([
         check(MOD1),
         check(MOD2),
         check(MOD3),
         check(REM1),
         check(REM2),
         check(REM3),
       ]));
 }