Beta 4: invalid context(this) of mixins when used vuex and custom mixin

[armano2]

Version

5.0.0-beta.4

Browser and OS info

Windows 10 / Firefox

Steps to reproduce

import Vue from "vue";
import Vuex from "vuex";

function mixin(_Vue) {
  _Vue.mixin({
    computed: {
      test() {
        if (this.$options.test) {
          return this.$options.test;
        }
        throw new Error("test is not defined");
      }
    }
  });
}

Vue.use(mixin);
Vue.use(Vuex);

/* eslint-disable no-new */
new Vue({
  el: "#app",
  test: 1,
  store: new Vuex.Store({
    state: {
      a: 1
    }
  }),
  computed: {
    foo() {
      return this.$store.state.a;
    }
  },
  template: `<div id="app">
    <h1>{{ test }}</h1>
    <h2>{{ foo }}</h2>
  </div>`
});

What is expected?

No error

What is actually happening?

test is not defined

---

DevTools are changing context (this) of mixins when store is registered as Vuex.Store instance.
This issue is present only in beta branch of devTools.

This issue is not present when instead of instance of store i provide object.

  store: {
    state: {
      a: 1
    }
  },

EDIT:

i did small simplification of reproduce code

[Akryum]

Closed in #904