* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

Our style guide says the k's are not necessary, and it seems like a good idea to make all the code be consistent on this.

Only naming changes to private vars: no logic changes.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

* PageView ballistics overshoot the page on some devices

On some devices, such as Cupertino “Plus”-sized devices, scrolling left on the first page of a PageView will overshoot the first page and land on the second page.

The issue is that applyContentDimensions incorrectly detects a content size change due to a floating point comparison on certain screen sizes (18257.400000000005 vs 18257.4)

To fix this, perform a nearEqual comparison in applyContentDimensions.

* Apply style changes to nearEqual for code review feedback.

`stiffness` is the name of a spring's `k` input on [iOS](https://developer.apple.com/documentation/quartzcore/caspringanimation), [Android](https://developer.android.com/topic/libraries/support-library/preview/spring-animation.html), and the [Web](https://github.com/skevy/wobble/blob/bbc0f831e234f2d24fb905f2ec278c134d69ed68/src/index.js#L11-L22).  To ensure the API is familiar to and easily understood by developers coming from other platforms, Flutter should follow this convention as well.

This is a minimally-breaking change.  Across [all of GitHub](https://github.com/search?l=Dart&q=springConstant&type=Code&utf8=%E2%9C%93), there are only 2 uses of the `springConstant` API (in Mondrian).  Those can be easily changed to use this name.

Closes #11684

* Add tolerance argument to FrictionSimulation.
* Change FrictionSimulation.through to setting tolerance
  via the constructor rather than afterwards.
* Allow SimulationGroup constructor to take tolerance argument.
* Add a toString for SpringDescription.
* Add a toString for SpringSimulation.
* Push this change to BouncingScrollSimulation.

Covers lib/ in package:flutter.

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`.

Rather that importing `package:newton/newton.dart` you can
`import package:flutter/physics.dart`.

Fixes #2441