Fix dartium constructor for KeyboardEvent.

tests/html/keyboard_event_test.dart

 library KeyboardEventTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 // Test that we are correctly determining keyCode and charCode uniformly across
 // browsers.
 
 main() {
 
   useHtmlConfiguration();
 
   keydownHandlerTest(KeyEvent e) {
     expect(e.charCode, 0);
   }
 
+  test('keyboardEvent constructor', () {
+    var event = new KeyboardEvent('keyup', document.window);
+  });
   test('keys', () {
     // This test currently is pretty much a no-op because we
     // can't (right now) construct KeyboardEvents with specific keycode/charcode
     // values (a KeyboardEvent can be "init"-ed but not all the information can
     // be programmatically populated. It exists as an example for how to use
     // KeyboardEventController more than anything else.
     var controller = new KeyboardEventController.keydown(document.window);
     var func = keydownHandlerTest;
     controller.add(func);
     document.window.on.keyDown.add((e) => print('regular listener'), false);
   });
 }