Move unittest from lib to pkg.

tests/html/cross_frame_test.dart

 #library('CrossFrameTest');
-#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('contentWindow', () {
       final iframe = new Element.tag('iframe');
       document.body.nodes.add(iframe);
       final frameWindow = iframe.contentWindow;
 
       // Ensure that the frame's document is inaccessible via window.
       expect(() => frameWindow.document, throws);
     });
 
   test('contentDocument', () {
       final iframe = new Element.tag('iframe');
       document.body.nodes.add(iframe);
 
       // Ensure that the frame's document is inaccessible.
       expect(() => iframe.contentDocument, throws);
     });
 }