fix vuejs#9046

虞上将 · 虞上将 · commit c60fb7f38823 · 2018-11-20T17:16:28.000+08:00
diff --git a/src/core/observer/dep.js b/src/core/observer/dep.js
@@ -56,7 +56,7 @@ Dep.target = null
 const targetStack = []
 
 export function pushTarget (_target: ?Watcher) {
-  if (Dep.target) targetStack.push(Dep.target)
+  if (Dep.target !== null) targetStack.push(Dep.target)
   Dep.target = _target
 }
 

Original file line number	Diff line number	Diff line change
`@@ -56,7 +56,7 @@ Dep.target = null`
`56`	`56`	`const targetStack = []`
`57`	`57`
`58`	`58`	`export function pushTarget (_target: ?Watcher) {`
`59`		`- if (Dep.target) targetStack.push(Dep.target)`
	`59`	`+ if (Dep.target !== null) targetStack.push(Dep.target)`
`60`	`60`	`Dep.target = _target`
`61`	`61`	`}`
`62`	`62`