Deprecate MouseEvent.elementTo and elementFrom since they have the same

tests/html/mouse_event_test.dart

Index: tests/html/mouse_event_test.dart
diff --git a/tests/html/mouse_event_test.dart b/tests/html/mouse_event_test.dart
index dcd442ff833f063a219cc9beb170173611f455bd..38792c27257a0093da2c6c9f518ccfba2a4e401f 100644
--- a/tests/html/mouse_event_test.dart
+++ b/tests/html/mouse_event_test.dart
@@ -13,5 +13,8 @@ main() {
   test('relatedTarget', () {
     var event = new MouseEvent('mouseout');
     expect(event.relatedTarget, isNull);
+
+    event = new MouseEvent('mouseout', relatedTarget: document.body);
+    expect(event.relatedTarget, document.body);
   });
 }