godoc: use virtual file system when generating package synopses

Fix for godoc on app engine.

R=dsymonds
CC=golang-dev
https://golang.org/cl/4922050

src/cmd/godoc/dirtrees.go

@@ -117,7 +117,7 @@ func (b *treeBuilder) newDirTree(fset *token.FileSet, path, name string, depth i
 			// though the directory doesn't contain any real package files - was bug)
 			if synopses[0] == "" {
 				// no "optimal" package synopsis yet; continue to collect synopses
-				file, err := parser.ParseFile(fset, filepath.Join(path, d.Name()), nil,
+				file, err := parseFile(fset, filepath.Join(path, d.Name()),
 					parser.ParseComments|parser.PackageClauseOnly)
 				if err == nil {
 					hasPkgFiles = true

src/cmd/godoc/parser.go

@@ -17,18 +17,18 @@ import (
 	"path/filepath"
 )
 
-func parseFiles(fset *token.FileSet, filenames []string) (pkgs map[string]*ast.Package, first os.Error) {
-	pkgs = make(map[string]*ast.Package)
-	for _, filename := range filenames {
+func parseFile(fset *token.FileSet, filename string, mode uint) (*ast.File, os.Error) {
 	src, err := fs.ReadFile(filename)
 	if err != nil {
-			if first == nil {
-				first = err
-			}
-			continue
+		return nil, err
 	}
+	return parser.ParseFile(fset, filename, src, mode)
+}
 
-		file, err := parser.ParseFile(fset, filename, src, parser.ParseComments)
+func parseFiles(fset *token.FileSet, filenames []string) (pkgs map[string]*ast.Package, first os.Error) {
+	pkgs = make(map[string]*ast.Package)
+	for _, filename := range filenames {
+		file, err := parseFile(fset, filename, parser.ParseComments)
 		if err != nil {
 			if first == nil {
 				first = err