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

tests/html/hidden_dom_2_test.dart

-#library('HiddenDom2Test');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library HiddenDom2Test;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 // Test that the dart:html API does not leak native jsdom methods:
 //   appendChild operation.
 
 main() {
   useHtmlConfiguration();
 
   test('test1', () {
     document.body.elements.add(new Element.html(r'''
 <div id='div1'>
@@ skipping 24 matching lines @@
     action();
   } catch (e) {
     threw = true;
     ex = e;
   }
   if (!threw)
     expect(false, isTrue, reason: 'Action should have thrown exception');
 
   expect(ex, isNoSuchMethodError);
 }