refactor: use faster key check

packages/core/src/componentProxy.ts

@@ -1,5 +1,5 @@
 import { ComponentInstance } from './component'
-import { isString, isFunction } from '@vue/shared'
+import { isFunction, isReservedKey } from '@vue/shared'
 
 const bindCache = new WeakMap()
 
@@ -37,7 +37,7 @@ const renderProxyHandlers = {
     ) {
       // computed
       return target._computedGetters[key]()
-    } else {
+    } else if (key[0] !== '_') {
       if (__DEV__ && !(key in target)) {
         // TODO warn non-present property
       }
@@ -57,7 +57,7 @@ const renderProxyHandlers = {
     receiver: any
   ): boolean {
     if (__DEV__) {
-      if (isString(key) && key[0] === '$') {
+      if (isReservedKey(key)) {
         // TODO warn setting immutable properties
         return false
       }

packages/core/src/componentState.ts

@@ -1,7 +1,6 @@
 import { ComponentInstance } from './component'
 import { observable } from '@vue/observer'
-
-const internalRE = /^_|^\$/
+import { isReservedKey } from '@vue/shared'
 
 export function initializeState(instance: ComponentInstance) {
   const { data } = instance.$options
@@ -18,7 +17,7 @@ export function extractInitializers(
   const keys = Object.keys(instance)
   for (let i = 0; i < keys.length; i++) {
     const key = keys[i]
-    if (!internalRE.test(key)) {
+    if (!isReservedKey(key)) {
       data[key] = (instance as any)[key]
     }
   }