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

tests/html/storage_test.dart

-#library('StorageTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library StorageTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   test('GetItem', () {
     final value = window.localStorage['does not exist'];
     expect(value, isNull);
   });
   test('SetItem', () {
     final key = 'foo';
     final value = 'bar';
     window.localStorage[key] = value;
     final stored = window.localStorage[key];
     expect(stored, value);
   });
 }