async invoke(identifier, event) {
    const func = this._getFunction(identifier);
    if (!func) {
      throw new Error(`Error: Cannot find function ${identifier} in template.`);
    }
    debug(`Found serverless function in template: ${JSON.stringify(func)}`);
    console.info(`Invoking function ${identifier} (${func.runtime}).`);

    const config = this._getInvokeConfig(func);
    await this._runtime.invoke(config, event, this._debugInfo);
  }
Invoke a function with specified event.

@param identifier the identifier of the function.
@param event the event which triggers the function.
@returns {Promise.<void>}