fix: allow plugins to be re-installed in localVue instance

closes vuejs#406

Author: kellym

src/create-local-vue.js

@@ -32,6 +32,7 @@ function createLocalVue (): Component {
   instance.options._base = instance
 
   // compat for vue-router < 2.7.1 where it does not allow multiple installs
+  instance._installedPlugins = []
   const use = instance.use
   instance.use = (plugin, ...rest) => {
     if (plugin.installed === true) {

test/specs/create-local-vue.spec.js

@@ -1,4 +1,5 @@
 import { createLocalVue } from '~vue-test-utils'
+import Vue from 'vue'
 import Vuex from 'vuex'
 import Vuetify from 'vuetify'
 import VueRouter from 'vue-router'
@@ -113,6 +114,23 @@ describe('createLocalVue', () => {
     localVue.use(Vuetify)
   })
 
+  it('installs plugin into local Vue regardless of previous install in Vue', () => {
+    let installCount = 0
+
+    class Plugin {}
+    Plugin.install = function (_Vue) {
+      expect(_Vue._installedPlugins.indexOf(Plugin)).to.equal(-1)
+      installCount++
+    }
+
+    Vue.use(Plugin)
+    const localVue = createLocalVue()
+    localVue.use(Plugin)
+
+    expect(localVue._installedPlugins.indexOf(Plugin)).to.equal(0)
+    expect(installCount).to.equal(2)
+  })
+
   it('has an errorHandler', () => {
     const localVue = createLocalVue()