Release 0.12.0-beta.0.

This is a beta release since all of the major foundational work is now
in place.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1039033002

pubspec.yaml

 name: unittest
-version: 0.12.0-alpha.1
+version: 0.12.0-beta.0
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/unittest

test/util/string_literal_iterator_test.dart

@@ -2,6 +2,8 @@
 // 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.
 
+@TestOn("vm")
+
 import 'package:analyzer/analyzer.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/src/util/string_literal_iterator.dart';