events: fix 'removeListener' event after #once

When a user listens for an event using #once, their listener is wrapped
in a function which will remove it after being called. While this was
accounted for with the 'newListener' event, 'removeListener' would
receive the wrapped function.

Fixes nodejs#5551

lib/events.js

@@ -300,7 +300,8 @@ EventEmitter.prototype.removeListener =
         else {
           delete events[type];
           if (events.removeListener)
-            this.emit('removeListener', type, listener);
+            this.emit('removeListener', type,
+                      listener.listener ? listener.listener : listener);
         }
       } else if (typeof list !== 'function') {
         position = -1;
@@ -329,7 +330,8 @@ EventEmitter.prototype.removeListener =
         }
 
         if (events.removeListener)
-          this.emit('removeListener', type, listener);
+          this.emit('removeListener', type,
+                    listener.listener ? listener.listener : listener);
       }
 
       return this;

test/parallel/test-event-emitter-remove-listeners.js

@@ -102,3 +102,19 @@ e6.emit('hello');
 
 // Interal listener array [listener3]
 e6.emit('hello');
+
+const e7 = new events.EventEmitter();
+e7.on('hello', listener1);
+e7.once('hello', listener2);
+e7.once('removeListener', common.mustCall(function(name, cb) {
+  assert.equal(name, 'hello');
+  assert.equal(cb, listener2);
+  assert.deepEqual([listener1], e7.listeners('hello'));
+  e7.once('removeListener', common.mustCall(function(name, cb) {
+    assert.equal(name, 'hello');
+    assert.equal(cb, listener1);
+    assert.deepEqual([], e7.listeners('hello'));
+  }));
+  e7.removeListener('hello', listener1);
+}));
+e7.emit('hello');