Move unittest from lib to pkg.

tests/html/history_test.dart

 #library('HistoryTest');
-#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('History', () {
     window.history.pushState(null, document.title, '?foo=bar');
     expect(window.history.length, equals(2));
     window.history.back();
     expect(window.location.href.endsWith('foo=bar'), isTrue);
 
     window.history.replaceState(null, document.title, '?foo=baz');
     expect(window.history.length, equals(2));
     expect(window.location.href.endsWith('foo=baz'), isTrue);
   });
 }