events: add type checks to once

Also cleanup unnecessary use of "self" since it will always be called
using .apply() from emit.

lib/events.js

@@ -165,18 +165,18 @@ EventEmitter.prototype.addListener = function(type, listener) {
 EventEmitter.prototype.on = EventEmitter.prototype.addListener;
 
 EventEmitter.prototype.once = function(type, listener) {
-  if ('function' !== typeof listener) {
+  if (typeof type !== 'string')
-    throw TypeError('.once only takes instances of Function');
+    throw TypeError('type must be a string');
+  if (typeof listener !== 'function')
+    throw TypeError('listener must be a function');
+
+  function g() {
+    this.removeListener(type, g);
+    listener.apply(this, arguments);
   }
 
-  var self = this;
-  function g() {
-    self.removeListener(type, g);
-    listener.apply(this, arguments);
-  };
-
   g.listener = listener;
-  self.on(type, g);
+  this.on(type, g);
 
   return this;
 };