checkers: fix for Go 1.4

The read-only flag has changed value in go 1.4.
Unfortunately we still need this hack.
It is possible that a future Go change might
render it unnecessary. Issue 8965 (https://code.google.com/p/go/issues/detail?id=8965)
tracks the issue.

Could you add a link to the bug in comments here so we know what it's for