Duplicated anonymous interfaces caused nondeterministic build.
The fix is CL 129515. This CL adds a test.

Updates #27013.

Change-Id: I6b7e1bbfc943c22e8e6f32c145f7aebb567cef15
Reviewed-on: https://go-review.googlesource.com/129680
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>