Update tests/html to use r'' strings

tests/html/hidden_dom_1_test.dart

 #library('HiddenDom1Test');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 // Test that the dart:html API does not leak native jsdom methods:
 //   onfocus setter.
 
 main() {
   useHtmlConfiguration();
 
   test('test1', () {
-    document.body.elements.add(new Element.html(@'''
+    document.body.elements.add(new Element.html(r'''
 <div id='div1'>
 Hello World!
 </div>'''));
     Element e = document.query('#div1');
     Expect.isTrue(e != null);
 
     checkNoSuchMethod(() { confuse(e).onfocus = null; });
   });
 
 }
@@ skipping 12 matching lines @@
   try {
     action();
   } catch (e) {
     ex = e;
   }
   if (ex === null)
     Expect.fail('Action should have thrown exception');
 
   Expect.isTrue(ex is NoSuchMethodError, 'ex is NoSuchMethodError');
 }