Move unittest from lib to pkg.

tests/html/htmlaudioelement_test.dart

 #library('AudioElementTest');
-#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('constructorTest1', () {
       var audio = new AudioElement();   // would be new Audio() in JS
       Expect.isTrue(audio != null);
       Expect.isTrue(audio is AudioElement);
     });
 
   test('constructorTest2', () {
       var audio = new AudioElement('hahaURL');
       Expect.isTrue(audio != null);
       Expect.isTrue(audio is AudioElement);
       Expect.isTrue(audio.src.indexOf('hahaURL') >= 0);
     });
 }