Move unittest from lib to pkg.

tests/html/storage_test.dart

 #library('StorageTest');
-#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('GetItem', () {
     final value = window.localStorage['does not exist'];
     Expect.isNull(value);
   });
   test('SetItem', () {
     final key = 'foo';
     final value = 'bar';
     window.localStorage[key] = value;
     final stored = window.localStorage[key];
     Expect.equals(value, stored);
   });
 }