improve DOM listener update performance

src/platforms/web/runtime/modules/events.js

@@ -1,33 +1,33 @@
-// skip type checking this file because we need to attach private properties
-// to elements
+/* @flow */
 
 import { updateListeners } from 'core/vdom/helpers/index'
 
-function updateDOMListeners (oldVnode, vnode) {
+let target: HTMLElement
+
+function add (event: string, handler: Function, once: boolean, capture: boolean) {
+  if (once) {
+    const oldHandler = handler
+    handler = function (ev) {
+      remove(event, handler, capture)
+      arguments.length === 1
+        ? oldHandler(ev)
+        : oldHandler.apply(null, arguments)
+    }
+  }
+  target.addEventListener(event, handler, capture)
+}
+
+function remove (event: string, handler: Function, capture: boolean) {
+  target.removeEventListener(event, handler, capture)
+}
+
+function updateDOMListeners (oldVnode: VNodeWithData, vnode: VNodeWithData) {
   if (!oldVnode.data.on && !vnode.data.on) {
     return
   }
   const on = vnode.data.on || {}
   const oldOn = oldVnode.data.on || {}
-  const add = vnode.elm._v_add || (
-    vnode.elm._v_add = (event, handler, once, capture) => {
-      if (once) {
-        const oldHandler = handler
-        handler = function (ev) {
-          remove(event, handler, capture)
-          arguments.length === 1
-            ? oldHandler(ev)
-            : oldHandler.apply(null, arguments)
-        }
-      }
-      vnode.elm.addEventListener(event, handler, capture)
-    }
-  )
-  const remove = vnode.elm._v_remove || (
-    vnode.elm._v_remove = (event, handler, capture) => {
-      vnode.elm.removeEventListener(event, handler, capture)
-    }
-  )
+  target = vnode.elm
   updateListeners(on, oldOn, add, remove, vnode.context)
 }