This CL includes changes from:

http://golang.org/cl/9805
http://golang.org/cl/9806
http://golang.org/cl/9901

Change-Id: I1f41a8215f9f760c0d3b84596e37bf48bf4c9bc2
Reviewed-on: https://go-review.googlesource.com/10132Reviewed-by: Rob Pike <r@golang.org>