HTML elements unsupported in component is

[CyberAP]

Version

3.0.0-alpha.9

Reproduction link

https://codesandbox.io/s/clever-banach-vutvf

Steps to reproduce

1. Create a component element with is attribute set to an HTML element string.

What is expected?

If no component is to be found it should simply use that string as a tag name. Or alternatively check if that string is a valid HTML built-in tag name.

What is actually happening?

Vue tries to find a component with such a name and fails to do so with a warning:

[Vue warn]: Failed to resolve component: span 
  at <App> "" 
  at <Anonymous> (Root) 

[Vue warn]: Invalid vnode type when creating vnode: undefined. 
  at <App> "" 
  at <Anonymous> (Root)