fix partial resolution in children

Author: yyx990803

src/element-directives/partial.js

@@ -44,7 +44,7 @@ module.exports = {
   },
 
   insert: function (id) {
-    var partial = this.vm.$options.partials[id]
+    var partial = _.resolveAsset(this.vm.$options, 'partials', id)
     _.assertAsset(partial, 'partial', id)
     if (partial) {
       var frag = templateParser.parse(partial, true)