Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/js_constant_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() {
     JS('', '#.toString()', -5);
     // absent: "5.toString"
     // present: "(-5).toString"
   }
 """;
 
 main() {
   Future check(String test) {
     Uri uri = new Uri(scheme: 'dart', path: 'test');
     var compiler = compilerFor(test, uri, expectedErrors: 0);
     return compiler.run(uri).then((_) {
       var element = findElement(compiler, 'main');
       var backend = compiler.backend;
       String generated = backend.getGeneratedCode(element);
       checkerForAbsentPresent(test)(generated);
     });
   }
 
   asyncTest(() => Future.wait([
         check(TEST_1),
       ]));
 }