dashboard/buildlet: darwin support

Update golang/go#9495

Change-Id: I732cfdee952ad3bf0b3411d0ce45723900acedb4
Reviewed-on: https://go-review.googlesource.com/2472
Reviewed-by: Andrew Gerrand <adg@golang.org>

dashboard/buildlet/Makefile

@@ -12,3 +12,7 @@ buildlet.plan9-386: buildlet.go
 buildlet.windows-amd64: buildlet.go
 	GOOS=windows GOARCH=amd64 go build -o $@ --tags=buildlet
 	cat $@ | (cd ../coordinator/buildongce && go run create.go --write_object=go-builder-data/$@)
+
+buildlet.darwin-amd64: buildlet.go
+	GOOS=darwin GOARCH=amd64 go build -o $@ --tags=buildlet
+	cat $@ | (cd ../coordinator/buildongce && go run create.go --write_object=go-builder-data/$@)

dashboard/buildlet/buildlet.go

@@ -48,6 +48,12 @@ var (
 )
 
 func defaultListenAddr() string {
+	if runtime.GOOS == "darwin" {
+		// Darwin will never run on GCE, so let's always
+		// listen on a high port (so we don't need to be
+		// root).
+		return ":5936"
+	}
 	if metadata.OnGCE() {
 		// In production, default to
 		return ":80"