fix(runtime-core): avoid mutating EMPTY_ARR when setting dev root (#2419)

also freeze EMPTY_ARR in dev

fix #2413

packages/vue/__tests__/index.spec.ts

@@ -1,3 +1,4 @@
+import { EMPTY_ARR } from '@vue/shared'
 import { createApp, ref, nextTick, reactive } from '../src'
 
 describe('compiler + runtime integration', () => {
@@ -281,4 +282,14 @@ describe('compiler + runtime integration', () => {
     await nextTick()
     expect(container.innerHTML).toBe(`<div>2<div>1</div></div>`)
   })
+
+  // #2413
+  it('EMPTY_ARR should not change', () => {
+    const App = {
+      template: `<div v-for="v of ['a']">{{ v }}</div>`
+    }
+    const container = document.createElement('div')
+    createApp(App).mount(container)
+    expect(EMPTY_ARR.length).toBe(0)
+  })
 })