Describe your environment

• Operating System version: NA
• Firebase SDK version: 4.2
• Firebase Product: messaging

Describe the problem

Actual:

There's no addEventListener() method on the firebase.messaging() object.

Expected:

The following code outputs "MESSAGE 1" and "MESSAGE 2" when a message is received:

firebase.messaging().addEventListener("message", () => console.log("MESSAGE 1"));
firebase.messaging().addEventListener("message", () => console.log("MESSAGE 2"));

Discussion:

The method names onMessage() and onTokenRefresh() suggest that firebase.messaging.Messaging implements the EventTarget interface. However, this is not the case, which impairs the DX, as well as making it impossible to easily add multiple listeners for the same event, remove listeners, etc.