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

tests/html/history_test.dart

-#library('HistoryTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library HistoryTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   test('History', () {
     window.history.pushState(null, document.title, '?foo=bar');
     expect(window.history.length, equals(2));
     window.history.back();
     expect(window.location.href.endsWith('foo=bar'), isTrue);
 
     window.history.replaceState(null, document.title, '?foo=baz');
     expect(window.history.length, equals(2));
     expect(window.location.href.endsWith('foo=baz'), isTrue);
   });
 }