* Upgrade packages to pull in newest petitparser with Dart 2 fixes.

* Don't allow Dart 2 failures.

* Address new lints discovered by updated analyzer.

* work around const lints

* update nonconst doc

Make regular Alignment values look more like enumProperties while
leaving more complex AlignmentDirectional values unchanged.

Alignment will eventually replace FractionalOffset.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.