Cannot reproduce original problem. Compiler internals
have changed enough such that this appears to work now.

Restore original test (exported interfaces), but also
keep version of the test using non-exported interfaces.

Fixes #15596.

Change-Id: Idb32da80239963242bd5d1609343c80f19773b0c
Reviewed-on: https://go-review.googlesource.com/43622
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>