utf8_test.BenchmarkRuneCountTenASCIIChars	20000000	       108 ns/op
utf8_test.BenchmarkRuneCountTenJapaneseChars	10000000	       199 ns/op
utf8_test.BenchmarkEncodeASCIIRune	500000000	         6 ns/op
utf8_test.BenchmarkEncodeJapaneseRune	100000000	        10 ns/op
utf8_test.BenchmarkDecodeASCIIRune	100000000	        13 ns/op
utf8_test.BenchmarkDecodeJapaneseRune	100000000	        21 ns/op

R=gri
CC=golang-dev
https://golang.org/cl/161049