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

tests/html/exceptions_test.dart

-#library('ExceptionsTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library ExceptionsTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   test('DOMException', () {
     try {
       window.webkitNotifications.createNotification('', '', '');
     } on DOMException catch (e) {
       expect(e.code, DOMException.SECURITY_ERR);
       expect(e.name, 'SECURITY_ERR');
       expect(e.message, 'SECURITY_ERR: DOM Exception 18');
     }
   });
   test('EventException', () {
     final event = new Event('Event');
     // Intentionally do not initialize it!
     try {
       document.$dom_dispatchEvent(event);
     } on EventException catch (e) {
       expect(e.code, EventException.UNSPECIFIED_EVENT_TYPE_ERR);
       expect(e.name, 'UNSPECIFIED_EVENT_TYPE_ERR');
       expect(e.message, 'UNSPECIFIED_EVENT_TYPE_ERR: DOM Events Exception 0');
     }
   });
 }