derp/derphttp: don't link websockets other than on GOOS=js

Or unless the new "ts_debug_websockets" build tag is set.

Updates #1278

Change-Id: Ic4c4f81c1924250efd025b055585faec37a5491d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derphttp/derphttp_test.go

@@ -17,7 +17,9 @@ import (
 
 	"tailscale.com/derp"
 	"tailscale.com/net/netmon"
+	"tailscale.com/tstest/deptest"
 	"tailscale.com/types/key"
+	"tailscale.com/util/set"
 )
 
 func TestSendRecv(t *testing.T) {
@@ -485,3 +487,23 @@ func TestProbe(t *testing.T) {
 		}
 	}
 }
+
+func TestDeps(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "darwin",
+		GOARCH: "arm64",
+		BadDeps: map[string]string{
+			"github.com/coder/websocket": "shouldn't link websockets except on js/wasm",
+		},
+	}.Check(t)
+
+	deptest.DepChecker{
+		GOOS:   "darwin",
+		GOARCH: "arm64",
+		Tags:   "ts_debug_websockets",
+		WantDeps: set.Of(
+			"github.com/coder/websocket",
+		),
+	}.Check(t)
+
+}