Example of output:

goroutine 4 [sleep for 3 min]:
time.Sleep(0x34630b8a000)
        src/pkg/runtime/time.goc:31 +0x31
main.func·002()
        block.go:16 +0x2c
created by main.main
        block.go:17 +0x33

Full program and output are here:
http://play.golang.org/p/NEZdADI3Td

Fixes #6809.

R=golang-codereviews, khr, kamil.kisiel, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/50420043