Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/interop_anonymous_unreachable_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 tests.dart2js.interop_anonymous_unreachable_test;
 
-import 'dart:async';
-
 import 'package:test/test.dart';
 import 'compiler_helper.dart';
 
 main() {
   test("unreachable code doesn't crash the compiler", () async {
     // This test is a regression for Issue #24974
     String generated = await compile(
         """
         import 'package:js/js.dart';
 
@@ skipping 148 matching lines @@
 
       generated = await compile(program2, returnAll: true);
       expect(generated.contains("UniqueLongNameForTesting_A"), isTrue);
       // This extra check is to make sure that we don't include HTMLAudioElement
       // just because of the is-check. It is optimized away in this case because
       // we believe it was never instantiated.
       expect(generated.contains("HTMLAudioElement"), isFalse);
     });
   });
 }