Add HttpStatus codes api seen in dart html library

[rutvik110]

Issue:

Dart html library exports HttpStatus abstract class that defines various http status codes. I can't find a replacement for this in the web package. Currently, the HttpStatus is part of darts internal apis so one can't directly import it in their project but this api is exported only through dart html and io libraries. As web is replacing the html library eventually, addition of this api would be quite handy.

Proposal:

Add HttpStatus api similar to one exported from html library. I believe one can just copy the api from dart sdk which should be good enough. I'm ready to put up a PR on this.