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

tests/html/async_window_test.dart

-#library('AsyncWindowTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library AsyncWindowTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   test('Window.setTimeout', () {
     window.setTimeout(expectAsync0((){}), 10);
   });
   test('Window.setInterval', () {
     int counter = 0;
     int id = null;
     id = window.setInterval(expectAsync0(() {
       if (counter == 3) {
         counter = 1024;
         window.clearInterval(id);
         // Wait some more time to be sure callback won't be invoked any more.
         window.setTimeout(expectAsync0((){}), 50);
         return;
       }
       // As callback should have been cleared on 4th invocation, counter
       // should never be greater than 3.
       assert(counter < 3);
       counter++;
     }, 3), 10);
   });
 }