Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/type_mask_test.dart

Index: tests/compiler/dart2js/type_mask_test.dart
diff --git a/tests/compiler/dart2js/type_mask_test.dart b/tests/compiler/dart2js/type_mask_test.dart
index 7cbe9e4f52c1e31921aa1600064178e33245e2e9..fa7d108fddd4e40901740a6e521e0a48cf13b285 100644
--- a/tests/compiler/dart2js/type_mask_test.dart
+++ b/tests/compiler/dart2js/type_mask_test.dart
@@ -2,11 +2,11 @@
 // 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 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
-import "package:async_helper/async_helper.dart";
+import 'package:compiler/src/types/types.dart';
+
 import 'compiler_helper.dart';
-import 'parser_helper.dart';
-import "package:compiler/src/types/types.dart";
 
 const String CODE = """
 class A {}