Update tests/html to use r'' strings

tests/html/hidden_dom_2_test.dart

 #library('HiddenDom2Test');
 #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:
 //   appendChild operation.
 
 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');
-    Element e2 = new Element.html(@"<div id='xx'>XX</div>");
+    Element e2 = new Element.html(r"<div id='xx'>XX</div>");
     Expect.isTrue(e != null);
 
     checkNoSuchMethod(() { confuse(e).appendChild(e2); });
 
   });
 }
 
 class Decoy {
   void appendChild(x) { throw 'dead code'; }
 }
@@ skipping 10 matching lines @@
     action();
   } catch (e) {
     threw = true;
     ex = e;
   }
   if (!threw)
     Expect.fail('Action should have thrown exception');
 
   Expect.isTrue(ex is NoSuchMethodError, 'ex is NoSuchMethodError');
 }