Restructure pkg/unittest and pkg/webdriver to follow the pub conventions.

tests/html/htmlaudioelement_test.dart

-#library('AudioElementTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library AudioElementTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
 
   useHtmlConfiguration();
 
   var isAudioElement =
       predicate((x) => x is AudioElement, 'is an AudioElement');
 
   test('constructorTest1', () {
       var audio = new AudioElement();   // would be new Audio() in JS
       expect(audio, isNotNull);
       expect(audio, isAudioElement);
     });
 
   test('constructorTest2', () {
       var audio = new AudioElement('hahaURL');
       expect(audio, isNotNull);
       expect(audio, isAudioElement);
       expect(audio.src.indexOf('hahaURL'), greaterThanOrEqualTo(0));
     });
 }