_getEngineInstance({ name, instanceId }) {
    let engineInstances = this._engineInstances;
    return engineInstances[name] && engineInstances[name][instanceId];
  }
Gets the instance of an Engine with the specified name and instanceId.

@private
@method _getEngineInstance
@param {Object} engineInfo
@param {String} engineInfo.name
@param {String} engineInfo.instanceId
@return {EngineInstance}