refactor: avoid using immutable object for $slot

libs like Avoiraz assumes $slots is mutable, need backwards
compat.

src/core/instance/render-helpers/resolve-slots.js

@@ -1,7 +1,6 @@
 /* @flow */
 
 import type VNode from 'core/vdom/vnode'
-import { emptyObject } from 'core/util/index'
 
 /**
  * Runtime helper for resolving raw children VNodes into a slot object.
@@ -11,7 +10,7 @@ export function resolveSlots (
   context: ?Component
 ): { [key: string]: Array<VNode> } {
   if (!children || !children.length) {
-    return emptyObject
+    return {}
   }
   const slots = {}
   for (let i = 0, l = children.length; i < l; i++) {

src/core/vdom/helpers/normalize-scoped-slots.js

@@ -1,16 +1,11 @@
 /* @flow */
 
-import { emptyObject } from 'core/util/index'
-
 export function normalizeScopedSlots (
   slots: { [key: string]: Function } | void,
   normalSlots: { [key: string]: Array<VNode> }
 ): any {
   let res
   if (!slots) {
-    if (normalSlots === emptyObject) {
-      return emptyObject
-    }
     res = {}
   } else if (slots._normalized) {
     return slots
@@ -19,14 +14,12 @@ export function normalizeScopedSlots (
     for (const key in slots) {
       res[key] = normalizeScopedSlot(slots[key])
     }
-    res._normalized = true
   }
   // expose normal slots on scopedSlots
-  if (normalSlots !== emptyObject) {
-    for (const key in normalSlots) {
-      res[key] = () => normalSlots[key]
-    }
+  for (const key in normalSlots) {
+    res[key] = () => normalSlots[key]
   }
+  res._normalized = true
   return res
 }