cmd/gotype: make it compile against Go 1.8 and earlier

types.SizesFor was introduced for Go 1.9. Need to provide
a local implementation to have gotype build against earlier
versions.

Fixes golang/go#19545.

Change-Id: I6fdbe414e6574eda00c01295b666230daff2dfdc
Reviewed-on: https://go-review.googlesource.com/38157
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
3 files changed