godoc: show all top-level decls for (fake) package builtin

R=r, adg
CC=golang-dev
https://golang.org/cl/4904045

src/cmd/godoc/godoc.go

@@ -767,6 +767,10 @@ func serveFile(w http.ResponseWriter, r *http.Request) {
 const fakePkgFile = "doc.go"
 const fakePkgName = "documentation"
 
+// Fake relative package path for built-ins. Documentation for all globals
+// (not just exported ones) will be shown for packages in this directory.
+const builtinPkgPath = "builtin/"
+
 type PageInfoMode uint
 
 const (
@@ -941,7 +945,10 @@ func (h *httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	relpath := r.URL.Path[len(h.pattern):]
 	abspath := absolutePath(relpath, h.fsRoot)
-	mode := exportsOnly
+	var mode PageInfoMode
+	if relpath != builtinPkgPath {
+		mode = exportsOnly
+	}
 	if r.FormValue("m") != "src" {
 		mode |= genDoc
 	}
@@ -964,7 +971,7 @@ func (h *httpHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		title = "Package " + info.PAst.Name.Name
 	case info.PDoc != nil:
 		switch {
-		case h.isPkg:
+		case info.IsPkg:
 			title = "Package " + info.PDoc.PackageName
 		case info.PDoc.PackageName == fakePkgName:
 			// assume that the directory name is the command name