Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/union_type_test.dart

Index: tests/compiler/dart2js/union_type_test.dart
diff --git a/tests/compiler/dart2js/union_type_test.dart b/tests/compiler/dart2js/union_type_test.dart
index 1b7e095b6db99f9d354a4586ac26c20da2e29dd5..c103d6fac87f4c0338c8db01a9e723f1ed4cb24d 100644
--- a/tests/compiler/dart2js/union_type_test.dart
+++ b/tests/compiler/dart2js/union_type_test.dart
@@ -2,13 +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:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
-import "compiler_helper.dart";
-import "parser_helper.dart";
-
+import "package:expect/expect.dart";
 import "package:compiler/src/types/types.dart";
-import "package:compiler/src/dart_types.dart";
+
+import "compiler_helper.dart";
 
 main() {
   MockCompiler compiler = new MockCompiler.internal(analyzeOnly: true);