Fix build after http://golang.org/cl/5297
The compiler was changed to not print implicit map capacity in error messages.

Change-Id: I852f668680c3c69c5eecc7964e46202a97014d6a
Reviewed-on: https://go-review.googlesource.com/8212Reviewed-by: Dmitry Vyukov <dvyukov@google.com>