Provider: Add extension on BuildContext

[shinayser]

Hey guys, playing with extensions of dart 2.6, I was thinking on a nice one:

extension ProviderExtensions<T> on BuildContext {
    T provide<T>() => Provider.of<T>(this);
}

Then you could use it like:

  var person = context.provide<Person>();

Would be a nice addition after the extensions functions hit stable :)
What you think?

[rrousselGit]

Hello!

I'd personally wait to see what Flutter does with extensions, to make sure that provider follows the same prototype/naming convention used by the SDK.

The implementation is easy enough that anybody can copy-paste it on their project, and the gain is minimalist anyway.
So that can definitely wait a bit IMO.

cc @felangel