MaterialPageRoute transitions are now defined by the Theme. Added (optional) support for Android P style page transitions.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

The average frame time of page transitions on Moto G4 is now very
close to 16ms (the last 10 measurements on our dashboard are
between 15.5ms to 16.7ms and half of them are below 16ms).

It is now much faster than when we disabled it (which was at about
35ms). So I think that we should be able to enable it by default.
I'll leave the flag there until we implement the retained rendering
to bring the frame time comfortably below 16ms.

See https://github.com/flutter/flutter/issues/13736

Semantics object support for edge triggered semantics 

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.

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

* enable lint prefer_asserts_in_initializer_lists

* enable --assert-initializer

This reverts commit e7061698.

This causes preview-dart-2 test failures.

* Roll engine

* Move assert requiring 'this' from initializer to constructor body

* Update test expected message

* Relax regexp to allow wider array of dart uris

Previously, we used `Alignment`, which was difficult to understand. Now,
we just use an `Offset` scaled to the child's size, which is much easier
to understand.

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.

* 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

* link more explicitly from route to navigator

* fix from review

* make @immutable const

* fix build

* 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

Remove intermediate animation listener

* 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 @required when there's an assert not null

* address review comments

* Add FadeTransition to Android page transition

* Also move starting point up

* Add test

* Use const fractional offset

Covers lib/ in package:flutter.