New a11y traversal:

- sort direct sibling SemanticsNodes only
- use new sorting algorithm
- implement RTL
- test semantics in traversal order by default
- add AppBar traversal test
- breaking: remove nextNodeId/previousNodeId from the framework
- breaking: remove DebugSemanticsDumpOrder.geometricOrder

Updates scaffold to expose bottom padding to the associated
BottonNavigationBar (if present). BottomNavigationBar is now responsible
for adjusting itself to account for bottom padding.

This change is necessary to support the updated BottomNavigationBar
layout for the iPhone X.

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._

This updates the bottom navigation bar to be able to handle more general widgets in the place of the label in the bottom navigation bar, so that Text with a textScaleFactor larger than 1.0 will behave nicely in a bottom navigation bar.

It also means that other widgets given instead of a Text widget for the label will work more predictably.

I also vastly simplified the layout logic, eliminating many computations that were not needed, and refactored the build function to use a separate private navigation tile widget.

Also, the color splash animations were coming from the wrong location (they were coming from far to the right of the touched widget), so that works as specified now.

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.

This widget makes it easier to animated Positioned widgets with
awareness of the ambient Directionality.

Fixes #11998

Move some Cupertino colors to a common file. 

Create a CupertinoTabBar widget to mimic iOS looks

Moved everything icons related except the material icon button and the materialicon font list to widget

Fixes https://github.com/flutter/flutter/issues/9342

I can't figure out if this is genius or a giant hack.

This lets you use DefaultTextStyle.merge and IconTheme.merge without
specifying a BuildContext. It automatically merges in at the
appropriate place in the tree using a Builder widget.

* 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

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

* add @required when there's an assert not null

* address review comments

These symbols were not intended to be public. Also, remove some bogus dartdocs
for PhysicalModeLayer.

Covers lib/ in package:flutter.

* rename DestinationLabel and labels properties to BottomNavigationBarItem and items

* update/fix comments

* grammatical changes

* add myself to AUTHORS

Changed DefaultTextStyle to consider the TextTheme defined in the context for Bottom Nav Bar (#7042)

* Changed DefaultTextStyle to consider the TextTheme defined in the context

* added regression test for bottom nav bar text themes

* added second regression test for fixed navbar

This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.

Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.

Fixes #6960
Fixes #5169

The circle's offset represents the position of a navigation button within the
row of buttons.  Previously, this offset had been applied to the entire width
of the bottom bar, not just the button region.

Fixes https://github.com/flutter/flutter/issues/6046

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

Changed the fixed type to better support the dark theme.

When switching from shifting to fixed, the BottomNavigationBar no
longer displays the old color of the background.