Adding polyfills for MouseEvent.offsetX and MouseEvent.offsetY so they work on FF.

tests/html/event_test.dart

Index: tests/html/event_test.dart
diff --git a/tests/html/event_test.dart b/tests/html/event_test.dart
index e3a2e6766104fec42883b57b53df5890b7f783d9..d3e759aa187a9ae866a65794c25de3406d2cd0ac 100644
--- a/tests/html/event_test.dart
+++ b/tests/html/event_test.dart
@@ -119,6 +119,8 @@ main() {
     Expect.equals(3, ev.screenY);
     Expect.equals(4, ev.clientX);
     Expect.equals(5, ev.clientY);
+    Expect.equals(4, ev.offsetX);  // Same as clientX.
+    Expect.equals(5, ev.offsetY);  // Same as clientY.
     Expect.equals(6, ev.button);
     Expect.isTrue(ev.ctrlKey);
     Expect.isTrue(ev.altKey);