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

tests/html/cache_test.dart

-#library('CacheTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library CacheTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   test('ApplicationCache', () {
     DOMApplicationCache appCache = window.applicationCache;
     expect(cacheStatusToString(appCache.status), equals("UNCACHED"));
   });
 }
 
 String cacheStatusToString(int status) {
   switch (status) {
     case DOMApplicationCache.UNCACHED: // UNCACHED == 0
       return 'UNCACHED';
     case DOMApplicationCache.IDLE: // IDLE == 1
       return 'IDLE';
     case DOMApplicationCache.CHECKING: // CHECKING == 2
       return 'CHECKING';
     case DOMApplicationCache.DOWNLOADING: // DOWNLOADING == 3
       return 'DOWNLOADING';
     case DOMApplicationCache.UPDATEREADY:  // UPDATEREADY == 4
       return 'UPDATEREADY';
     case DOMApplicationCache.OBSOLETE: // OBSOLETE == 5
       return 'OBSOLETE';
     default:
       return 'UNKNOWN CACHE STATUS';
   };
 }