Move unittest from lib to pkg.

tests/html/exceptions_test.dart

 #library('ExceptionsTest');
-#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('DOMException', () {
     try {
       window.webkitNotifications.createNotification('', '', '');
     } catch (DOMException e) {
       Expect.equals(DOMException.SECURITY_ERR, e.code);
       Expect.equals('SECURITY_ERR', e.name);
       Expect.equals('SECURITY_ERR: DOM Exception 18', e.message);
     }
   });
   test('EventException', () {
     final event = new Event('Event');
     // Intentionally do not initialize it!
     try {
       document.$dom_dispatchEvent(event);
     } catch (EventException e) {
       Expect.equals(EventException.UNSPECIFIED_EVENT_TYPE_ERR, e.code);
       Expect.equals('UNSPECIFIED_EVENT_TYPE_ERR', e.name);
       Expect.equals('UNSPECIFIED_EVENT_TYPE_ERR: DOM Events Exception 0', e.message);
     }
   });
 }