function _subtreeFilter(file, scope) {
        if (scope) {
            if (scope.isDirectory) {
                // Dirs always have trailing slash, so we don't have to worry about being
                // a substring of another dir name
                return file.fullPath.indexOf(scope.fullPath) === 0;
            } else {
                return file.fullPath === scope.fullPath;
            }
        }
        return true;
    }
Checks that the file matches the given subtree scope. To fully check whether the file
should be in the search set, use _inSearchScope() instead - a supserset of this.

@param {!File} file
@param {?FileSystemEntry} scope Search scope, or null if whole project
@return {boolean}