fix(node): crash when calling listeners() for event with no listeners

Author: alxhub

Diff for: lib/common/utils.ts

@@ -253,6 +253,9 @@ export function makeZoneAwareListeners(fnName: string) {
   return function zoneAwareEventListeners(self: any, args: any[]) {
     const eventName: string = args[0];
     const target = self || _global;
+    if (!target[EVENT_TASKS]) {
+      return [];
+    }
     return target[EVENT_TASKS]
         .filter(task => task.data.eventName === eventName)
         .map(task => task.data.handler);

Diff for: test/node/events.spec.ts

@@ -58,4 +58,9 @@ describe('nodejs EventEmitter', () => {
     });
     expect(emitter.listeners('test')).toEqual([expectZoneA, shouldNotRun]);
   });
+  it('should return empty array when an event has no listeners', () => {
+    zoneA.run(() => {
+      expect(emitter.listeners('test')).toEqual([]);
+    });
+  })
 });