fix linting issues

src/core/instance/render.js

@@ -4,7 +4,7 @@ import config from '../config'
 import VNode, { emptyVNode } from '../vdom/vnode'
 import { normalizeChildren } from '../vdom/helpers'
 import {
-  warn, bind, remove, extend, isObject, toObject,
+  warn, bind, remove, isObject, toObject,
   nextTick, resolveAsset, renderString
 } from '../util/index'
 

test/unit/features/directives/bind.spec.js

@@ -155,7 +155,7 @@ describe('Directive v-bind', () => {
   })
 
   it('warn expect object', () => {
-    const vm = new Vue({
+    new Vue({
       template: '<input v-bind="test">',
       data: {
         test: 1

test/unit/features/directives/style.spec.js

@@ -1,5 +1,4 @@
 import Vue from 'vue'
-import { extend } from 'shared/util'
 
 function checkPrefixedProp (prop) {
   var el = document.createElement('div')