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.

This cleans up lerp, lerpFrom, lerpTo, and scale methods, and their
documentation.

Fixes https://github.com/flutter/flutter/issues/12377.

Fixes the remaining known issues with widgets supporting RTL.

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.

* 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.

* Refactor CupertinoScaffold

* Rename rootTabPageBuilder to tabBuilder

* fix tab transparency padding

* add a CupertinoTab

* Add default background color

* Add a bunch of examples

* A bunch of tests

* Refactor CupertinoScaffold

* Rename rootTabPageBuilder to tabBuilder

* fix tab transparency padding

* Add default background color

* review notes

* fix test

* review

* Rename CupertinoTab to CupertinoTabView

* remove final ; in sample code for analyzer

* Refactor CupertinoScaffold

* Rename rootTabPageBuilder to tabBuilder

* fix tab transparency padding

* Add default background color

* review notes

* fix test

* Make iOS back gesture triggerable from the edge only.

* change to positioned

Add Diagnosticable base class and documentation

It was 8.0. It's now arbitrarily 18.0.

Changing this required adjusting some tests. Adjusting the tests
required debugging the tests. Debugging the tests required some tools
to help debugging gesture recognizers and gesture arenas, so I added
some. It also required updating some toString() methods which resulted
in some changes to the tree diagnostics logic.

Also I cleaned up some docs while I was at it.

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

* 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

This patch finishes documenting all the symbols except for those related to
scrolling.

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

* 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