async atomicOperationAcquireMutex_(mutexIdentifier) {
    const operationId = ++this.operationId_;
    await this.mutex_.acquire(mutexIdentifier);
    if (operationId != this.operationId_) {
      this.mutex_.release();
      return true;
    }
    return false;
  }
Tries to acquire the mutex, and then returns if the operation should end
early due to someone else starting a mutex-acquiring operation.
Meant for operations that can't be interrupted midway through (e.g.
everything but load).
@param {string} mutexIdentifier
@return {!Promise<boolean>} endEarly If false, the calling context will
  need to release the mutex.
@private