【报Bug】slot数据涉及计算时报错

操作步骤：

1. 定义一个组件page，组件代码是循环一个list，设置一个slot，引用page组件的地方，实现slot每行的部分

   <view v-for="(item, index) in pageInfo.list" :key="index">  
   <slot :item="item"></slot>  
   </view>

2. 引用page组件，实现slot部分

   <page ref="orderListPage" :url="pageInfo.url" :rows="pageInfo.rows" @complete="handlePageComplete">  
   <template v-slot="{item}">  
       <view class="list-item">  
           <image class="item-image" src="../../static/images/example.jpg" mode=""></image>  
           <view class="item-content">  
               <text class="title">{{item['taskId']}}</text>  
               <text class="small-title">xx</text>  
           </view>  
           <view class="item-complete">已完成{{item['requirement'] | precent(item['requirement'])}}</view>  
           <view class="item-complete">已完成{{Math.floor(item['requirement'])}}</view>  
       </view>  
   </template>  
   </page>

3. 对比，不使用slot的数据，filter和数学计算都能正常显示

   <view class="list-item">  
   <image class="item-image" src="../../static/images/example.jpg" mode=""></image>  
   <view class="item-content">  
       <text class="title">xx</text>  
       <text class="small-title">xx</text>  
   </view>  
   <view class="item-complete">已完成{{test.a | precent(test.b)}}</view>  
   <view class="item-complete">已完成{{Math.floor(test.a)}}</view>  
   </view>

预期结果：

已完成{{item['requirement'] | precent(item['requirement'])}} 使用filter的部分能正确显示
已完成{{Math.floor(item['requirement'])}} 数学计算的部分能正确显示

实际结果：

报错
VM18979:1 [Vue warn]: Property or method "item" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.