_anonymous is an internal option.

src/directives/if.js

@@ -43,7 +43,7 @@ module.exports = {
     this.childVM = this.vm.$addChild({
       el: this.el,
       parent: this.vm,
-      anonymous: true
+      _anonymous: true
     })
   },
 

src/instance/init.js

@@ -43,7 +43,7 @@ exports._init = function (options) {
 
   // anonymous instances are created by flow-control
   // directives such as v-if and v-repeat
-  this._isAnonymous = options.anonymous
+  this._isAnonymous = options._anonymous
 
   // merge options.
   this.$options = mergeOptions(