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

tests/html/cross_frame_test.dart

-#library('CrossFrameTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library CrossFrameTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   var isWindow = predicate((x) => x is Window, 'is a Window');
   var isLocalWindow = predicate((x) => x is LocalWindow, 'is a LocalWindow');
   var isLocation = predicate((x) => x is Location, 'is a Location');
   var isLocalLocation =
       predicate((x) => x is LocalLocation, 'is a LocalLocation');
   var isHistory = predicate((x) => x is History, 'is a History');
@@ skipping 51 matching lines @@
 
       // Valid methods.
       frameHistory.forward();
 
       expect(() => frameHistory.length, throws);
 
       final frameParentHistory = iframe.contentWindow.parent.history;
       expect(frameParentHistory, isLocalHistory);
     });
 }