fix: actually disable dep collection when invoking lifecycle hooks (vuejs#9095)

fix vuejs#9046

Author: ysj16

src/core/observer/dep.js

@@ -55,11 +55,12 @@ export default class Dep {
 Dep.target = null
 const targetStack = []
 
-export function pushTarget (_target: ?Watcher) {
-  if (Dep.target) targetStack.push(Dep.target)
-  Dep.target = _target
+export function pushTarget (target: ?Watcher) {
+  targetStack.push(target)
+  Dep.target = target
 }
 
 export function popTarget () {
-  Dep.target = targetStack.pop()
+  targetStack.pop()
+  Dep.target = targetStack[targetStack.length - 1]
 }

test/unit/features/instance/methods-lifecycle.spec.js

@@ -1,4 +1,5 @@
 import Vue from 'vue'
+import Dep from 'core/observer/dep'
 
 describe('Instance methods lifecycle', () => {
   describe('$mount', () => {
@@ -32,6 +33,26 @@ describe('Instance methods lifecycle', () => {
       expect(vm.$el.tagName).toBe('DIV')
       expect(vm.$el.textContent).toBe('hi')
     })
+
+    it('Dep.target should be undefined in lifecycle', () => {
+      const vm = new Vue({
+        template: '<div><my-component></my-component></div>',
+        components: {
+          myComponent: {
+            template: '<div>hi</div>',
+            mounted () {
+              const _msg = this.msg
+              expect(Dep.target).toBe(undefined)
+            },
+            computed: {
+              msg () {
+                return 1
+              }
+            }
+          }
+        }
+      }).$mount()
+    })
   })
 
   describe('$destroy', () => {