Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/library_resolution_test.dart

 // Copyright (c) 2014, 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.
 
 /// Check that relative URIs are resolved against the canonical name of a
 /// library. This only matters for dart:-libraries, so this test mocks up two
 /// dart:-libraries.
 
-import "dart:io";
-
 import "dart:async";
 
 import "memory_source_file_helper.dart";
 
 import "package:async_helper/async_helper.dart";
 
 import 'package:expect/expect.dart' show Expect;
 
 import 'package:compiler/compiler_new.dart';
 
@@ skipping 71 matching lines @@
   var libraries =
       await compiler.libraryLoader.loadLibrary(Uri.parse("dart:m_o_c_k_1"));
   await checkLibraries(libraries);
   asyncSuccess(null);
 }
 
 const Map MEMORY_SOURCE_FILES = const {
   "mock1.dart": "library mock1; import 'mock2.dart';",
   "mock2.dart": "library mock2;",
 };