perf(v-for): use faster array population in renderList

packages/runtime-core/src/helpers/renderList.ts

@@ -5,14 +5,16 @@ export function renderList(
   source: any,
   renderItem: (value: any, key: string | number, index?: number) => VNodeChild
 ): VNodeChild[] {
-  let ret: VNodeChild[] = []
+  let ret: VNodeChild[]
   if (isArray(source) || isString(source)) {
+    ret = new Array(source.length)
     for (let i = 0, l = source.length; i < l; i++) {
-      ret.push(renderItem(source[i], i))
+      ret[i] = renderItem(source[i], i)
     }
   } else if (typeof source === 'number') {
+    ret = new Array(source)
     for (let i = 0; i < source; i++) {
-      ret.push(renderItem(i + 1, i))
+      ret[i] = renderItem(i + 1, i)
     }
   } else if (isObject(source)) {
     if (source[Symbol.iterator as any]) {
@@ -26,5 +28,5 @@ export function renderList(
       }
     }
   }
-  return ret
+  return ret!
 }