Revert "fix #4041, warn overriding Vue's internal methods (#4111)"

This reverts commit 4078ce9.

Author: yyx990803

Diff for: src/core/instance/state.js

@@ -21,8 +21,6 @@ import {
   noop
 } from '../util/index'
 
-import BuiltinVue from '../index'
-
 export function initState (vm: Component) {
   vm._watchers = []
   initProps(vm)
@@ -145,16 +143,12 @@ function initMethods (vm: Component) {
   if (methods) {
     for (const key in methods) {
       vm[key] = methods[key] == null ? noop : bind(methods[key], vm)
-      if (process.env.NODE_ENV !== 'production') {
-        methods[key] == null && warn(
+      if (process.env.NODE_ENV !== 'production' && methods[key] == null) {
+        warn(
           `method "${key}" has an undefined value in the component definition. ` +
           `Did you reference the function correctly?`,
           vm
         )
-        hasOwn(BuiltinVue.prototype, key) && warn(
-          `Avoid overriding Vue's internal method "${key}".`,
-          vm
-        )
       }
     }
   }

Diff for: test/unit/features/options/methods.spec.js

@@ -24,14 +24,4 @@ describe('Options methods', () => {
     })
     expect(`method "hello" has an undefined value in the component definition`).toHaveBeenWarned()
   })
-
-  it('should warn overriding builtin methods', () => {
-    new Vue({
-      methods: {
-        $emit () {
-        }
-      }
-    })
-    expect(`Avoid overriding Vue's internal method "$emit".`).toHaveBeenWarned()
-  })
 })