Don't delete _i18n in beforeDestroy

src/mixin.js

@@ -127,8 +127,6 @@ export default {
         self._localeWatcher()
         delete self._localeWatcher
       }
-
-      self._i18n = null
     })
   }
 }

test/unit/component.test.js

@@ -135,8 +135,6 @@ describe('component translation', () => {
       assert.strictEqual(subChild2.textContent, 'sub-child2')
 
       vm.$destroy()
-    }).then(() => {
-      assert(vm.$i18n === null)
     }).then(done)
   })
 })

test/unit/issues.test.js

@@ -693,4 +693,22 @@ describe('issues', () => {
       assert.strictEqual(vm.$te('message.empty'), true)
     })
   })
+
+  describe('#879', () => {
+    it('$t should not throw when invoked on a destroyed component', async () => {
+      const vm = new Vue({
+        i18n: new VueI18n({ locale: 'en' }),
+        methods: {
+          test: async function () {
+            // Long running async method that terminates after the component is destroyed.
+            await new Promise((resolve) => setTimeout(resolve, 50))
+            this.$t('anything')
+          }
+        }
+      })
+      const promise = vm.test() // invocation before being destroyed
+      vm.$destroy()
+      await promise // should not throw
+    })
+  })
 })