[improvement] add constant of border (#4078)

src/sidebar-item/index.js

@@ -1,7 +1,8 @@
 import { createNamespace } from '../utils';
-import Info from '../info';
 import { ChildrenMixin } from '../mixins/relation';
 import { route, routeProps } from '../utils/router';
+import { BORDER } from '../utils/constant';
+import Info from '../info';
 
 const [createComponent, bem] = createNamespace('sidebar-item');
 
@@ -32,7 +33,7 @@ export default createComponent({
   render() {
     return (
       <a
-        class={[bem({ select: this.select }), 'van-hairline']}
+        class={[bem({ select: this.select }), BORDER]}
         onClick={this.onClick}
       >
         <div class={bem('text')}>