### What problem does this feature solve? How to iterate over Map and Set with `v-for` ? I want to use the key/index field or both.(ES6 Map/Set) ```html <mt-index-list> <mt-index-section v-for="(list, key) in serverData.indexMap" :index="key" :key="key"> <mt-cell title="Aaron"></mt-cell> </mt-index-section> </mt-index-list> ``` ### What does the proposed API look like? N/A <!-- generated by vue-issues. DO NOT REMOVE -->