use WidgetsBindingObserver as a mixin

[yjbanov]

This change is fully backwards-compatible. The reason I'm doing it is that Flutter is converting to the new mixin syntax, which forbids extending mixins.

See also: flutter/flutter#22290

[Hixie]

Hm, this is sad. That class was meant to be usable as a superclass too.

[bparrishMines]

LGTM

[yjbanov]

I'm going to submit this PR because it's non-breaking but will unblock the move to mixin syntax if we decide to use it for non-super-mixins too.

[yjbanov]

I filed dart-lang/language#31 for discussion.

[Hixie]

I really would rather we didn't do this.

[yjbanov]

Why not?

[yjbanov]

(I'm assuming you are referring to this PR and not flutter/flutter#22290 or dart-lang/language#31)

[Hixie]

Because it's ugly. :-)

We have an ordinary class, and we want to use it as an ordinary superclass... why would we want to use it as a mixin in this case?

WidgetsBindingObserver was intended to be a regular class that in some more advanced cases might be used as a mixin or an interface, something that Dart supports out of the box and is idiomatic. Why would we shy away from that?

[yjbanov]

I think dart-lang/language#31 (comment) would solve this issue if accepted.