fix: clean up target variables to avoid memory leaks (#6932)

fix #6931

src/core/instance/events.js

@@ -19,7 +19,7 @@ export function initEvents (vm: Component) {
   }
 }
 
-let target: Component
+let target: any
 
 function add (event, fn, once) {
   if (once) {
@@ -40,6 +40,7 @@ export function updateComponentListeners (
 ) {
   target = vm
   updateListeners(listeners, oldListeners || {}, add, remove, vm)
+  target = undefined
 }
 
 export function eventsMixin (Vue: Class<Component>) {

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

@@ -26,7 +26,7 @@ function normalizeEvents (on) {
   }
 }
 
-let target: HTMLElement
+let target: any
 
 function createOnceHandler (handler, event, capture) {
   const _target = target // save current target element in closure
@@ -78,6 +78,7 @@ function updateDOMListeners (oldVnode: VNodeWithData, vnode: VNodeWithData) {
   target = vnode.elm
   normalizeEvents(on)
   updateListeners(on, oldOn, add, remove, vnode.context)
+  target = undefined
 }
 
 export default {

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

@@ -46,6 +46,7 @@ function updateDOMListeners (oldVnode: VNodeWithData, vnode: VNodeWithData) {
   const oldOn = oldVnode.data.on || {}
   target = vnode.elm
   updateListeners(on, oldOn, add, remove, vnode.context)
+  target = undefined
 }
 
 export default {