rank warning and error messages

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#20426

---

It would be great to sort errors by severity - all warnings are right now treated equal, but we probably want to rank them in some order.

This order might not be entirely static. For example, consider these 3 errors:
 a- there is an error of a broken import
 b- an warning in a template
 c- an warning that an element definition was not found.

I'd rank them in that order, because it's common that (c) is a consequence of (a). But if 'a' error is not there, I'd sort (c) before (b).