code: function findQualifiedChildren (instance) {
    return isQualified(instance)
      ? capture(instance)
      : findQualifiedChildrenFromList(instance.$children).concat(
        instance._vnode && instance._vnode.children
          // Find functional components in recursively in non-functional vnodes.
          ? flatten(instance._vnode.children.filter(child => !child.componentInstance).map(captureChild))
            // Filter qualified children.
            .filter(instance => isQualified(instance))
          : []
      )
  }
docs: Find qualified children from a single instance.
  If the instance itself is qualified, just return itself.
  This is ok because [].concat works in both cases.
  
  @param {Vue|Vnode} instance
  @return {Vue|Array}