Fixed an edge case in Tracker.nonreactive.

Author: mitar

src/core/observer/dep.js

@@ -49,7 +49,7 @@ Dep.target = null
 const targetStack = []
 
 export function pushTarget (_target: Watcher) {
-  if (Dep.target) targetStack.push(Dep.target)
+  targetStack.push(Dep.target)
   Dep.target = _target
 }
 

src/core/observer/watcher.js

@@ -91,7 +91,7 @@ export default class Watcher {
   /**
    * Evaluate the getter, and re-collect dependencies.
    */
-  get () {
+  get (dontCleanupDeps: ?boolean) {
     pushTarget(this)
     let value
     const vm = this.vm
@@ -110,7 +110,9 @@ export default class Watcher {
         traverse(value)
       }
       popTarget()
-      this.cleanupDeps()
+      if (!dontCleanupDeps) {
+        this.cleanupDeps()
+      }
     }
     return value
   }