Move unittest from lib to pkg.

tests/html/htmlelement_test.dart

 #library('ElementTest');
-#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('InnerHTML', () {
     Element element = new Element.tag('div');
     element.id = 'test';
     element.innerHTML = 'Hello World';
     document.body.nodes.add(element);
 
@@ skipping 68 matching lines @@
 
     Expect.equals('foo-value', div.dataAttributes.remove('foo'));
     Expect.equals(1, div.dataAttributes.length);
     Expect.isFalse(div.dataAttributes.isEmpty());
 
     div.dataAttributes.clear();
     Expect.equals(0, div.dataAttributes.length);
     Expect.isTrue(div.dataAttributes.isEmpty());
   });
 }