refactor(dart/transform): Use package:guinness in tests

`guinness` is a Dart port of Jasmine. Since the rest of Angular 2 uses
Jasmine, use it for the transformer too.

Closes #8

Closes #1037

Closes #1000

modules/angular2/test/transform/integration/all_tests.dart

@@ -5,7 +5,7 @@ import 'package:angular2/src/dom/html5lib_adapter.dart';
 import 'package:angular2/transformer.dart';
 import 'package:code_transformers/tests.dart';
 import 'package:dart_style/dart_style.dart';
-import 'package:unittest/unittest.dart';
+import 'package:guinness/guinness.dart';
 
 import '../common/read_file.dart';