json-1
cputime                  99600000     98600000      -1.00%
time                    100005493     98859693      -1.15%

garbage-1
cputime                  15760000     15440000      -2.03%
time                     15791759     15471701      -2.03%

LGTM=khr
R=golang-codereviews, gobot, khr, dave
CC=bradfitz, golang-codereviews, iant
https://golang.org/cl/57310043