fix(mergeOptions): skip mixins and extends if child is already merged

fixes vuejs#8865

Author: KaelWD

src/core/util/options.js

@@ -379,10 +379,10 @@ export function mergeOptions (
   normalizeInject(child, vm)
   normalizeDirectives(child)
   const extendsFrom = child.extends
-  if (extendsFrom) {
+  if (extendsFrom && !child._base) {
     parent = mergeOptions(parent, extendsFrom, vm)
   }
-  if (child.mixins) {
+  if (child.mixins && !child._base) {
     for (let i = 0, l = child.mixins.length; i < l; i++) {
       parent = mergeOptions(parent, child.mixins[i], vm)
     }

test/unit/features/global-api/extend.spec.js

@@ -71,6 +71,25 @@ describe('Global API: extend', () => {
     expect(calls).toEqual([1, 2, 3])
   })
 
+  it('should not merge nested mixins', () => {
+    const A = Vue.extend({
+      created: () => {}
+    })
+    const B = Vue.extend({
+      mixins: [A],
+      created: () => {}
+    })
+    const C = Vue.extend({
+      extends: B,
+      created: () => {}
+    })
+    const D = Vue.extend({
+      mixins: [C],
+      created: () => {}
+    })
+    expect(D.options.created.length).toBe(4)
+  })
+
   it('should merge methods', () => {
     const A = Vue.extend({
       methods: {