add addEventListener to eventsource (#25125)

types/eventsource/eventsource-tests.ts

@@ -4,6 +4,7 @@ const eventSource = new EventSource("http://foobar");
 eventSource.onmessage = (x: any) => {};
 eventSource.onerror = (x: any) => {};
 eventSource.onopen = (x: any) => {};
+eventSource.addEventListener = (type: string, x: any) => {};
 eventSource.close();
 
 import EventSourcePolyfill = require("eventsource/lib/eventsource-polyfill");
@@ -12,4 +13,5 @@ const eventSourcePolyfill = new EventSourcePolyfill("http://foobar");
 eventSourcePolyfill.onmessage = (x: any) => {};
 eventSourcePolyfill.onerror = (x: any) => {};
 eventSourcePolyfill.onopen = (x: any) => {};
+eventSourcePolyfill.addEventListener = (type: string, x: any) => {};
 eventSourcePolyfill.close();

types/eventsource/index.d.ts

@@ -16,6 +16,7 @@ declare class EventSource {
   onopen: EventListener;
   onmessage: EventListener;
   onerror: EventListener;
+  addEventListener(type: string, listener: EventListener): void;
   close(): void;
 }
 

types/eventsource/lib/eventsource-polyfill/index.d.ts

@@ -10,6 +10,7 @@ declare class EventSource {
   onopen: EventListener;
   onmessage: EventListener;
   onerror: EventListener;
+  addEventListener(type: string, listener: EventListener): void;
   close(): void;
 }