[release-branch.go1.4] cmd/api: make API check fail for undeclared API in release branch

We forgot to do the usual API review.
Make that not possible in the future.
I'll pull this change over to the main
branch too, but it's more important
(and only testable) here.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/185050043

src/cmd/api/goapi.go

@@ -283,7 +283,7 @@ func compareAPI(w io.Writer, features, required, optional, exception []string) (
 				delete(optionalSet, newFeature)
 			} else {
 				fmt.Fprintf(w, "+%s\n", newFeature)
-				if !*allowNew {
+				if !*allowNew || !strings.Contains(runtime.Version(), "devel") {
 					ok = false // we're in lock-down mode for next release
 				}
 			}
@@ -313,11 +313,15 @@ func fileFeatures(filename string) []string {
 	if err != nil {
 		log.Fatalf("Error reading file %s: %v", filename, err)
 	}
-	text := strings.TrimSpace(string(bs))
-	if text == "" {
-		return nil
+	lines := strings.Split(string(bs), "\n")
+	var nonblank []string
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+		if line != "" && !strings.HasPrefix(line, "#") {
+			nonblank = append(nonblank, line)
+		}
 	}
-	return strings.Split(text, "\n")
+	return nonblank
 }
 
 var fset = token.NewFileSet()