Moduralize the noop() function (vuejs#2834)

Currently the `noop` function is being declared and used randomly at
places. This commit moduralizes it (à la lodash) for more DRY code.

src/compiler/codegen.js

@@ -1,10 +1,11 @@
 import { genHandlers } from './events'
 import { ref } from './directives/ref'
 import { baseWarn } from './helpers'
+import { noop } from 'shared/util'
 
 const baseDirectives = {
   ref,
-  cloak: function () {} // noop
+  cloak: noop
 }
 
 // configurable state

src/core/instance/state.js

@@ -13,7 +13,8 @@ import {
   isArray,
   isPlainObject,
   bind,
-  validateProp
+  validateProp,
+  noop
 } from '../util/index'
 
 export function initState (vm) {
@@ -63,8 +64,6 @@ function initData (vm) {
   observe(data, vm)
 }
 
-function noop () {}
-
 function initComputed (vm) {
   const computed = vm.$options.computed
   if (computed) {

src/entries/web-runtime.js

@@ -6,6 +6,7 @@ import baseModules from 'core/vdom/modules/index'
 import platformModules from 'web/runtime/modules/index'
 import { query, isUnknownElement, isReservedTag } from 'web/util/index'
 import { inBrowser } from 'core/util/env'
+import { noop } from 'core/util/index'
 
 // install platform specific utils
 Vue.config.isUnknownElement = isUnknownElement
@@ -18,7 +19,7 @@ Vue.options.directives = platformDirectives
 const modules = baseModules.concat(platformModules)
 Vue.prototype.__patch__ = inBrowser
   ? createPatchFunction({ nodeOps, modules })
-  : function noop () {}
+  : noop
 
 // wrap mount
 Vue.prototype.$mount = function (el) {

src/shared/util.js

@@ -195,3 +195,8 @@ export function isPlainObject (obj) {
  * @return {Boolean}
  */
 export const isArray = Array.isArray
+
+/**
+ * Perform no operation.
+ */
+export function noop () {}