The new type was inheriting the tflagExtraStar from its prototype.

Fixes #15467

Change-Id: Ic22c2a55cee7580cb59228d52b97e1c0a1e60220
Reviewed-on: https://go-review.googlesource.com/22501Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>