net/http: apparent deadlock in TestServerGracefulClose on illumos-amd64 builder

[bcmills]

2019-12-16T20:38:31-f7f9866/illumos-amd64
2019-11-04T15:18:34-d3660e8/illumos-amd64
2019-10-04T20:09:41-60dc159/illumos-amd64-joyent

goroutine 7503 [IO wait, 2 minutes]:
internal/poll.runtime_pollWait(0xfffffbffeecafb48, 0x77, 0xc001c1dca0)
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/runtime/netpoll.go:203 +0x6d
internal/poll.(*pollDesc).wait(0xc0001ed118, 0x77, 0x470300, 0x4fc360, 0xffffffffffffffff)
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/internal/poll/fd_poll_runtime.go:87 +0x45
internal/poll.(*pollDesc).waitWrite(...)
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/internal/poll/fd_poll_runtime.go:96
internal/poll.(*FD).Write(0xc0001ed100, 0xc001b8e000, 0x50003b, 0x58c000, 0x0, 0x0, 0x0)
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/internal/poll/fd_unix.go:276 +0x290
net.(*netFD).Write(0xc0001ed100, 0xc001b8e000, 0x50003b, 0x58c000, 0xc000227f48, 0xc000054228, 0xc000054220)
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/net/fd_unix.go:220 +0x4f
net.(*conn).Write(0xc000180020, 0xc001b8e000, 0x50003b, 0x58c000, 0x0, 0x0, 0x0)
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/net/net.go:196 +0x8e
net/http_test.TestServerGracefulClose.func2(0xa5cc40, 0xc000180020, 0xc001b8e000, 0x50003b, 0x58c000, 0xc00032c3c0)
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/net/http/serve_test.go:3100 +0x5b
created by net/http_test.TestServerGracefulClose
	/var/tmp/workdir-host-illumos-amd64-jclulow/go/src/net/http/serve_test.go:3099 +0x35e

CC @bradfitz @ianlancetaylor @jclulow