Noticed this while closing tabs. Yesterday I thought I could
ignore this garbage and hope that a fix for issue 2205 handled
it, but I just realized that's the opposite case,
string->[]byte, whereas this is []byte->string.  I'm having a
hard time convincing myself that an Issue 2205-style fix with
static analysis and faking a string header would be safe in
all cases without violating the memory model (callee assumes
frozen memory; are there non-racy ways it could keep being
modified?)

R=dsymonds
CC=dave, gobot, golang-dev
https://golang.org/cl/6850067