This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.

The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.

* Make FadeTransition more efficient

* Update fade_transition_test.dart

* Update proxy_box.dart

Hopefully this will fix the performance regression in https://github.com/flutter/flutter/pull/13680 but with an easier way to flip the switch.

Fixes #12877 by reverting #9472

This reverts commit e73d4061.

It caused major performance regressions.

Fixes #12877 by reverting #9472

* upgrade to linter-0.1.35

* re-enable lint prefer_const_constructors

* address review comments

* started copying stuff into cupertino page route

* extracted from material page route. Ready for testing

* works with button and gesture

* tests and docs

* review notes

* review notes

...and remove some highly specialised methods now that they can just
be implemented directly by the previous callers.

Creates another Decoration for drawing outside the decorated box with a gradient to emulate the shadow.

Lets the cupertino transition page's background be transparent.

Fixes #9321

* more widget const constructors

* prefer const constructors

* address review comments

* Remove FadeTransition from _MountainViewFadeTransition.
* Update tests.

* Make Cupertino page transition elevation animated too

* Rename and change physical model to a decorated box

* Tests

* Add a comment

* still need to handle null in the tween somewhere

* nits

* Tweens evaluate to the actual begin/end instances. Let them be non-null

* Rename no decoration to none

* Scaffold appBar is-a PreferredSizeWidget, etc

* Updated

* Updated per review feedback

* Manually fix every use of Point.x and Point.y

Some of these were moved to dx/dy, but not all.

* Manually convert uses of the old gradient API

* Remove old reference to Point.

* Mechanical changes

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\bPoint[.]origin\b/Offset.zero/g'
git ls-files -z | xargs -0 sed -i 's/\bPoint[.]lerp\b/Offset.lerp/g'
git ls-files -z | xargs -0 sed -i 's/\bnew Point\b/new Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bconst Point\b/const Offset/g'
git ls-files -z | xargs -0 sed -i 's/\bstatic Point /static Offset /g'
git ls-files -z | xargs -0 sed -i 's/\bfinal Point /final Offset /g'
git ls-files -z | xargs -0 sed -i 's/^\( *\)Point /\1Offset /g'
git ls-files -z | xargs -0 sed -i 's/ui[.]Point\b/ui.Offset/g'
git ls-files -z | xargs -0 sed -i 's/(Point\b/(Offset/g'
git ls-files -z | xargs -0 sed -i 's/\([[{,]\) Point\b/\1 Offset/g'
git ls-files -z | xargs -0 sed -i 's/@required Point\b/@required Offset/g'
git ls-files -z | xargs -0 sed -i 's/<Point>/<Offset>/g'
git ls-files -z | xargs -0 sed -i 's/[.]toOffset()//g'
git ls-files -z | xargs -0 sed -i 's/[.]toPoint()//g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point, /show /g'
git ls-files -z | xargs -0 sed -i 's/\bshow Point;/show Offset;/g'

* Mechanical changes - dartdocs

I applied the following at the root of the Flutter repository:

git ls-files -z | xargs -0 sed -i 's/\ba \[Point\]/an [Offset]/g'
git ls-files -z | xargs -0 sed -i 's/\[Point\]/[Offset]/g'

* Further improvements and a test

* Fix minor errors from rebasing...

* Roll engine

Aggressively apply the const lint.

* Just realized the creation order is the other way around

* Works

* Revert previous partial solution

* Change pumpAndSettle to pump for gestures

* Moved stuff around yet

* Fix depedencies

* Add more dartdoc comments to packages

* Remove Cupertino dependency on material

* Removed mountain_view package and added page transition test

* Fix analyze warnings

* Remove commented code

* Partial solution. Still need to stop the animation on the previous page for modal

* Some review notes

* Move the cupertino back gesture controller’s lifecycle management back to its parent

* Reviews

* Add background color

* Directional curves, full screen transition

* Don’t perform the exit animation if the incoming page is a dialog

* It works!

* Test structures

* Add a bunch of more tests and fix the gallery

* One more comment

* Review notes

* final controller

* Use that sweet sweet `is!` keyword

* Play golf, because I’m bitter that there’s no nullable `as` or something in dart

* Remove a space

* Review notes

* Remove the last deprecated test

* Moved stuff around yet

* Fix depedencies

* Add more dartdoc comments to packages

* Remove Cupertino dependency on material

* Removed mountain_view package and added page transition test

* Fix analyze warnings

* Remove commented code

* Some review notes

* Move the cupertino back gesture controller’s lifecycle management back to its parent

* Reviews

* Add background color

* final controller

* Review notes

* Add FadeTransition to Android page transition

* Also move starting point up

* Add test

* Use const fractional offset