fix vuejs#4041, warn overriding Vue's internal methods

Author: HerringtonDarkholme

src/core/instance/state.js

@@ -21,6 +21,8 @@ import {
   noop
 } from '../util/index'
 
+import BuiltinVue from '../index'
+
 export function initState (vm: Component) {
   vm._watchers = []
   initProps(vm)
@@ -143,12 +145,17 @@ 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 "${key}" has an undefined value in the component definition. ` +
           `Did you reference the function correctly?`,
           vm
         )
+        hasOwn(BuiltinVue.prototype, key) && warn(
+          `You're overriding Vue's internal method "${key}". ` +
+          `Beware of misbehaviors.`,
+          vm
+        )
       }
     }
   }

test/unit/features/options/methods.spec.js

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