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

tests/html/htmlelement_test.dart

-#library('ElementTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library ElementTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   test('InnerHTML', () {
     Element element = new Element.tag('div');
     element.id = 'test';
     element.innerHTML = 'Hello World';
     document.body.nodes.add(element);
 
     element = document.query('#test');
@@ skipping 67 matching lines @@
 
     expect(div.dataAttributes.remove('foo'), 'foo-value');
     expect(div.dataAttributes.length, 1);
     expect(div.dataAttributes.isEmpty, isFalse);
 
     div.dataAttributes.clear();
     expect(div.dataAttributes.length, 0);
     expect(div.dataAttributes.isEmpty, isTrue);
   });
 }