cmd/vet: give warning for construct 'Println(os.Stderr, ...)'

also fixes this bug in net/http/httptest.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5654083

src/cmd/vet/print.go

@@ -207,7 +207,18 @@ func (f *File) checkPrintfVerb(call *ast.CallExpr, verb rune, flags []byte) {
 // call.Args[skip] is the first argument to be printed.
 func (f *File) checkPrint(call *ast.CallExpr, name string, skip int) {
 	isLn := strings.HasSuffix(name, "ln")
+	isF := strings.HasPrefix(name, "F")
 	args := call.Args
+	// check for Println(os.Stderr, ...)
+	if skip == 0 && !isF && len(args) > 0 {
+		if sel, ok := args[0].(*ast.SelectorExpr); ok {
+			if x, ok := sel.X.(*ast.Ident); ok {
+				if x.Name == "os" && strings.HasPrefix(sel.Sel.Name, "Std") {
+					f.Warnf(call.Pos(), "first argument to %s is %s.%s", name, x.Name, sel.Sel.Name)
+				}
+			}
+		}
+	}
 	if len(args) <= skip {
 		if *verbose && !isLn {
 			f.Badf(call.Pos(), "no args in %s call", name)

src/pkg/net/http/httptest/server.go

@@ -95,7 +95,7 @@ func (s *Server) Start() {
 	s.URL = "http://" + s.Listener.Addr().String()
 	go s.Config.Serve(s.Listener)
 	if *serve != "" {
-		fmt.Println(os.Stderr, "httptest: serving on", s.URL)
+		fmt.Fprintln(os.Stderr, "httptest: serving on", s.URL)
 		select {}
 	}
 }