Fired when a message is sent from another extension/app. Cannot be used in a content script.

class OnMessageExternalEvent {
 /**
  * The message sent by the calling script.
  * `optional`
  * 
  * The message sent by the calling script.
  */
 final dynamic message;

 final MessageSender sender;

 /**
  * Function to call (at most once) when you have a response. The argument
  * should be any JSON-ifiable object. If you have more than one `onMessage`
  * listener in the same document, then only one may send a response. This
  * function becomes invalid when the event listener returns, unless you return
  * true from the event listener to indicate you wish to send a response
  * asynchronously (this will keep the message channel open to the other end
  * until `sendResponse` is called).
  */
 final dynamic sendResponse;

 OnMessageExternalEvent(this.message, this.sender, this.sendResponse);
}