Coalesce set/get pairs into a tee.

Change-Id: I88ccdcb148465615437bebf24145e941a037e0a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/200357
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>