When running:
$ go test -race net/http
ThreadSanitizer says:

WARNING: DATA RACE
Write by goroutine 205:
  bufio.(*Reader).Read()
      src/pkg/bufio/bufio.go:152 +0x4b6
  io.(*LimitedReader).Read()
      src/pkg/io/io.go:406 +0x181
  net/http.(*body).Read()
      src/pkg/net/http/transfer.go:538 +0xd4
  io/ioutil.devNull.ReadFrom()
      src/pkg/io/ioutil/ioutil.go:138 +0x6f
  io/ioutil.devNull.ReadFrom·i()
      src/pkg/io/ioutil/ioutil.go:0 +0x52
  io.Copy()
      src/pkg/io/io.go:352 +0xc6
  net/http.(*body).Close()
      src/pkg/net/http/transfer.go:628 +0x170
  net/http.(*bodyEOFSignal).Close()
      src/pkg/net/http/transport.go:817 +0xb9
  net/http_test.func·082()
      src/pkg/net/http/transport_test.go:428 +0xb7

Previous write by goroutine 111:
  bufio.(*Reader).Read()
      src/pkg/bufio/bufio.go:152 +0x4b6
  io.(*LimitedReader).Read()
      src/pkg/io/io.go:406 +0x181
  net/http.(*body).Read()
      src/pkg/net/http/transfer.go:538 +0xd4
  io/ioutil.devNull.ReadFrom()
      src/pkg/io/ioutil/ioutil.go:138 +0x6f
  io/ioutil.devNull.ReadFrom·i()
      src/pkg/io/ioutil/ioutil.go:0 +0x52
  io.Copy()
      src/pkg/io/io.go:352 +0xc6
  net/http.(*body).Close()
      src/pkg/net/http/transfer.go:628 +0x170
  net/http.(*bodyEOFSignal).Close()
      src/pkg/net/http/transport.go:817 +0xb9
  net/http_test.func·082()
      src/pkg/net/http/transport_test.go:428 +0xb7

Goroutine 205 (running) created at:
  net/http_test.TestStressSurpriseServerCloses()
      src/pkg/net/http/transport_test.go:432 +0x369
  testing.tRunner()
      src/pkg/testing/testing.go:297 +0xc9

Goroutine 111 (running) created at:
  net/http_test.TestStressSurpriseServerCloses()
      src/pkg/net/http/transport_test.go:432 +0x369
  testing.tRunner()
      src/pkg/testing/testing.go:297 +0xc9

When devNull is used all goroutines write to the single blackHole slice. The race aside,
it has performance implications when several parallel goroutines write the same memory.