All strong mode cleaning of dart2js.

tests/compiler/dart2js/serialization/duplicate_library_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.
 
 library dart2js.serialization.duplicate_libraryc_test;
 
-import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common/names.dart';
-import 'package:compiler/src/compiler.dart';
-import 'package:compiler/src/filenames.dart';
 import '../memory_compiler.dart';
 import 'helper.dart';
-import 'test_data.dart';
 
 void main(List<String> args) {
   asyncTest(() async {
     SerializedData data =
         await serializeDartCore(arguments: new Arguments.from(args));
     Map<String, String> sourceFiles = data.toMemorySourceFiles();
     List<Uri> resolutionInputs = data.toUris();
     Uri extraUri = Uri.parse('memory:extraUri');
     sourceFiles[extraUri.path] = data.data;
     resolutionInputs.add(extraUri);
 
     DiagnosticCollector collector = new DiagnosticCollector();
     await runCompiler(
         entryPoint: Uris.dart_core,
         memorySourceFiles: sourceFiles,
         resolutionInputs: resolutionInputs,
         diagnosticHandler: collector,
         options: [Flags.analyzeAll]);
     Expect.isTrue(collector.errors.isNotEmpty,
         "Expected duplicate serialized library errors.");
   });
 }