Before
json.BenchmarkCodeEncoder  10  181232100 ns/op  10.71 MB/s
json.BenchmarkCodeMarshal  10  184578000 ns/op  10.51 MB/s

After:
json.BenchmarkCodeEncoder  10  146444000 ns/op  13.25 MB/s
json.BenchmarkCodeMarshal  10  151428500 ns/op  12.81 MB/s

R=rsc, r
CC=golang-dev
https://golang.org/cl/5416046