Move unittest from lib to pkg.

tests/html/messageevent_test.dart

 #library('SerializedScriptValueTest');
-#import('../../lib/unittest/unittest.dart');
-#import('../../lib/unittest/html_config.dart');
+#import('../../pkg/unittest/unittest.dart');
+#import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
   useHtmlConfiguration();
 
   test('MessageEvent.initMessageEvent', () {
       final event = document.$dom_createEvent('MessageEvent');
       event.initMessageEvent('type', false, true, 'data', 'origin', 'lastEventId', window, []);
       expect(event.type, equals('type'));
       expect(event.bubbles, isFalse);
       expect(event.cancelable, isTrue);
       expect(event.data, equals('data'));
       expect(event.origin, equals('origin'));
       expect(event.lastEventId, equals('lastEventId'));
       expect(event.source, same(window));
       // TODO(antonm): accessing ports is not supported yet.
   });
 }