sendCommand_(to, from, message) {
    const payload = { to, from, message, id: ++this.lastMessageId_ };
    if (this.debug_)
      console.log('\nSEND', payload);
    const result = new Promise((fulfill, reject) =>
      this.callbacks_.set(payload.id, {fulfill, reject}));
    this.routeMessage_(false, payload);
    return result;
  }
Sends message to the target handle and receive the response.

@param {!Object} payload
@return {!Promise<!Object>}