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

tests/html/window_open_test.dart

-#library('WindowOpenTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library WindowOpenTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   evaluateJavaScript(code) {
     final scriptTag = new Element.tag('script');
     scriptTag.innerHTML = code;
     document.body.nodes.add(scriptTag);
   }
   evaluateJavaScript('(testRunner || layoutTestController).setCanOpenWindows()');
 
   test('TwoArgumentVersion', () {
     Window win = window.open('../resources/pong.html', 'testWindow');
     closeWindow(win);
   });
   test('ThreeArgumentVersion', () {
     Window win = window.open("resources/pong.html", "testWindow", "scrollbars=yes,width=75,height=100");
     closeWindow(win);
   });
 }
 
 closeWindow(win) {
   win.close();
   doneHandler() {
     if (!win.closed) {
       window.setTimeout(expectAsync0(doneHandler), 1);
     }
   }
   window.setTimeout(expectAsync0(doneHandler), 1);
 }