static hasOnlyClosurePathsInConformance(filePath, contents) {
    const errors = [];
    let lineNo = 1;
    for (const line of contents) {
      if (CONFORMANCE_ALLOWLIST_REGEX.test(line)) {
        errors.push(`${filePath}: Non-Closure path found on line ${lineNo}`);
      }
      lineNo++;
    }
    return errors;
  }
Checks whether a conformance textproto file contains only Closure paths.
@param {string} filePath The path to conformance_proto.txt.
@param {!Array<string>} contents The contents of conformance_proto.txt.
@return {!Array<string>} A list of error strings. An empty list indicates
that the check passes.