static objectStringify_(obj) {
    // NOTE: This excludes prototype chain keys.  For now, this is intended for
    // anonymous objects only, so we don't care.  If that changes, go back to a
    // for-in loop.
    const keys = Object.keys(obj);
    // Alphabetically sort the keys, so they will be in a reliable order.
    keys.sort();

    const terms = [];
    for (const key of keys) {
      const escapedKey = JSON.stringify(key);
      const value = obj[key];
      if (value !== undefined) {
        const escapedValue =
            shaka.util.ObjectUtils.alphabeticalKeyOrderStringify(value);
        if (escapedValue) {
          terms.push(escapedKey + ':' + escapedValue);
        }
      }
    }
    return '{' + terms.join(',') + '}';
  }
Helper for alphabeticalKeyOrderStringify for objects.

@param {!Object} obj
@return {string}
@private