Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/import_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that the compiler can handle missing files used in imports, exports,
 // part tags or as the main source file.
 
 library dart2js.test.import;
 
-import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/diagnostics/messages.dart';
-import 'package:compiler/compiler.dart';
 import 'memory_compiler.dart';
 
 const MEMORY_SOURCE_FILES = const {
   'main.dart': '''
 
 library main;
 
 import 'dart:thisLibraryShouldNotExist';
 import 'package:thisPackageShouldNotExist/thisPackageShouldNotExist.dart';
 export 'foo.dart';
@@ skipping 64 matching lines @@
 }
 
 void main() {
   asyncTest(() async {
     await testEntryPointIsPart();
     await testImportPart();
     await testMissingImports();
     await testMissingMain();
   });
 }