before:
runtime_test.BenchmarkCallClosure        5000000               499 ns/op
runtime_test.BenchmarkCallClosure1       5000000               681 ns/op

after:
runtime_test.BenchmarkCallClosure       500000000                5 ns/op
runtime_test.BenchmarkCallClosure1       10000000              160 ns/op

R=rsc
CC=golang-dev
https://golang.org/cl/4515167