This matches the error message of cmd/compile (for assignments).

Change-Id: I42a428f5d72f034e7b7e97b090a929e317e812af
Reviewed-on: https://go-review.googlesource.com/38315
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>