Move unittest from lib to pkg.

tests/html/hidden_dom_2_test.dart

 #library('HiddenDom2Test');
-#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');
 
 // Test that the dart:html API does not leak native jsdom methods:
 //   appendChild operation.
 
 main() {
   useHtmlConfiguration();
 
   test('test1', () {
     document.body.elements.add(new Element.html(@'''
@@ skipping 25 matching lines @@
     action();
   } catch (var e) {
     threw = true;
     ex = e;
   }
   if (!threw)
     Expect.fail('Action should have thrown exception');
 
   Expect.isTrue(ex is NoSuchMethodException, 'ex is NoSuchMethodException');
 }