func (fc *FileCache) Line(filename string, line int) ([]byte, error) {
	if filepath.Ext(filename) != ".go" {
		return nil, nil
	}

	// Clean filenames returned by src.Pos.SymFilename()
	// or src.PosBase.SymFilename() removing
	// the leading src.FileSymPrefix.
	filename = strings.TrimPrefix(filename, src.FileSymPrefix)

	// Expand literal "$GOROOT" rewritten by obj.AbsFile()