Fix test to pass on IE, FF

tests/html/cross_frame_test.dart

 #library('CrossFrameTest');
 #import('../../pkg/unittest/unittest.dart');
 #import('../../pkg/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
   useHtmlConfiguration();
 
   final iframe = new Element.tag('iframe');
   document.body.nodes.add(iframe);
@@ skipping 31 matching lines @@
       expect(frameParentLocation is LocalLocation);
     });
 
   test('history', () {
       expect(window.history is LocalHistory);
       final frameHistory = iframe.contentWindow.history;
       expect(frameHistory is History);
       expect(frameHistory is! LocalHistory);
 
       // Valid methods.
-      frameHistory.back();
       frameHistory.forward();
 
       expect(() => frameHistory.length, throws);
 
       final frameParentHistory = iframe.contentWindow.parent.history;
       expect(frameParentHistory is LocalHistory);
     });
 }