wantSub := "\r\n--" + tt.b + "--\r\n"
			if got := b.String(); !strings.Contains(got, wantSub) {
				t.Errorf("expected %q in output. got: %q", wantSub, got)
			}
		}
	}
}

func TestWriterBoundaryGoroutines(t *testing.T) {
	// Verify there's no data race accessing any lazy boundary if it's used by
	// different goroutines. This was previously broken by
	// https://codereview.appspot.com/95760043/ and reverted in