This patch makes a number of changes to how you can configure a
Scrollable2:
- The ScrollPhysics is now responsible for creating the ScrollPosition.
You can override the ScrollPhysics by supplying a `physics` argument
to `Scrollable`, and the new physics you supply will be applied to
the default physics inherited from the ScrollBehavior.
- This patch removes the ScrollPosition/AbsoluteScrollPosition split as
all clients were operating in pixels anyway and the split made the
code very difficult to follow.
- ScrollPosition no longer depends directly on Scrollable2State.
Instead, it depends on an abstract interface that Scrollable2State
implements. This change has two benefits:
a) It removes the circular dependency between ScrollPosition and
Scrollable2State, which lets us split the code for these classes
(and several other classes that got wrapped up in that cycle) into
separate libraries for easier maintenance.
b) ScrollPosition is no longer bound to Scrollable2, which means you
could use the behavior machinery to drive other sorts of widgets.
For example, we could use it to drive Scrollabe1 if we wanted.
Also, make hasOneLineDescription more discerning.
Also, add a test for hasOneLineDescription.
Also, add a test for GravitySimulation, to test the toString.
It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/). We can also safely flip the bit on `avoid_return_types_on_setters`.
