This contains a breaking API change to `SelectionChangedHandler`.

This is part of making text input accessible on Andriod (https://github.com/flutter/flutter/issues/12785).

* Make drawer demo header not scroll

* Add test for gallery drawer demo and fix user accounts drawer header overflow

Summary:

- Add `key` field to `SemanticsNode`, while moving key into `foundation` library so it can be used by the render layer.
- Introduce `SemanticsProperties` and move many of the `Semantics` fields into it.
- Introduce `CustomPaintSemantics` - a `SemanticsNode` prototype created by `CustomPainter`.
- Introduce `semanticsBuilder` and `shouldRebuildSemantics` in `CustomerPainter`

**Breaking change**

The default `Semantics` constructor becomes non-const (due to https://github.com/dart-lang/sdk/issues/20962). However, a new `const Semantics.fromProperties` is added that still allowed creating constant `Semantics` widgets ([mailing list announcement](https://groups.google.com/forum/#!topic/flutter-dev/KQXBl2_1sws)).

Fixes https://github.com/flutter/flutter/issues/11791
Fixes https://github.com/flutter/flutter/issues/1666

Also, extra docs.

Fixes https://github.com/flutter/flutter/issues/13279
Fixes https://github.com/flutter/flutter/issues/2115

...and document what I did.

* ensures that only semantics boundaries will be added to owner._nodesNeedingSemantics as expected by compiler.
* no longer throws assert if markNeedsSemanticsUpdate is called on non-semantic-boundary render object with a non-semantic-boundary parent.
* Fixes #13109.
* removes onlyLocalUpdates from markNeedsSemanticsUpdate as its no longer needed.

* Implements FlatButton debugFillDescription

* Fix doc comment

* Override debugFillProperties instead of debugFillDescription

* Remove child debug property

* add an assert to validate the RefreshIndicator.onRefresh result

* add a test for RefreshIndicator.onRefresh assert

* switch to using FlutterError.reportError, FlutterErrorDetails, and FlutterError

This reverts commit e73d4061.

It caused major performance regressions.

Changed the ExpansionTile widget to have an optional value to
initialize the widget as collapsed or expanded.

The widget will be collapsed by default.

This fixes the popup menu code to do a better job of expanding
smoothly regardless of which side of the screen it's on. It still
results in a bidirection growth when positioned at the bottom of the
screen, so maybe we'll need to animate menus differently, but that's
a problem for another patch.

Also, improve some docs and provide RelativeRect.toSize which I needed
at one point while building this patch (though it didn't survive all
the way to the end).

Fixes #12877 by reverting #9472

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.

This fixes DropdownButtons to align their popups correctly in RTL as well.

Also while I was there I fixed the issue with text scale factor in the gallery.

Previously, ExpansionPanel would do weird things if interacted with
when it was already animating. This is fixed and there's now a test.

Also:

 * Minor fixes to make the gallery work in RTL, not that there's
   any way to see that without hard-coding the framework to RTL.
   But at least I'll be less annoyed when doing that.

 * Some trivial code and documentation cleanup.

This makes ListTile expand vertically when text is scaled, or really when whatever is placed inside it is larger than the available space.

In order for UnconstrainedBox to be useful here, I needed for it to only unconstrain the child Row in one dimension, so I added a "constrainedAxis" parameter to the UnconstrainedBox.

Also, changed one enum test to use a switch instead.

I modified the ListTile test to be more representative of the intention of the spec: we were testing with text in the leading and trailing sections, and the design wants icons there. Because there was leading text, and the dense mode only changes the font size on the text lines, the leading text was propping up the minimum size of the tile, making so that the test wasn't really testing any changes in dense mode.

UnconstrainedBox will allow its child to size itself as if it had no constraints, and then attempt to fit around that object, until its own constraints are exceeded, in which case it will clip and display an overflow warning.

I also factored out DebugOverflowIndicator, which will draw overflow indicators on containers which overflow but aren't expected to.

I'm not really sure how to test this without a reference test, since
eventually we want to move this to a font feature and thus the obvious
test (looking for the Transform widget) isn't really valid.

* enable lint prefer_asserts_in_initializer_lists

* enable --assert-initializer

* Surface shadowColor from RenderPhysicalModel to Material

* Fix typo in material_test

* Add nullability info to documentation

* Add support for animating elevation shadow color

* Add shadowColor to Material's debugFillProperties()

* Add missing default value for elevation in Material debugFillProperties()

* Add missing non-null asserts for animate flags in AnimatedPhysicalModel

* Add test for shadow color animating smoothly

* remove extracted methods

* lint

This fixes the assumption that InputDecorator had where it expected baseStyle to be a complete style. Now it merges the baseStyle with the subhead style instead of substituting it entirely.

This fixes #12832.

Adds `Scaffold#endDrawer` property to supply a second drawer to a Scaffold.

In order to allow chips to be properly drawn when they expand in size (without
using IntrinsicHeight), I needed a BoxDecoration shape that would be dependent
upon the rendered height of the widget. This seemed to be pretty generally
useful, so I added a new ShapeDecoration called StadiumBorder. It uses the
minimum dimension to adjust the BorderRadius of a rounded rect in the shape
decoration.

I also converted some uses of BoxShape to be case statements, updated the
chips to use the StadiumBorder decoration, and updated some of the metrics to match
the Material spec, as well as implementing lerping to and from StadiumBorder.

* alwaysUse24HourFormat in MediaQuery and time picker

* docs; dead code

* address some comments

* MaterialLocalizations.timeOfDayFormat is the single source of 24-hour-formattedness

* Make TimePickerDialog private again

* wire up MediaQueryData.fromWindow to Window

* Add acceptNotification parameter to RefreshIndicator and OverscrollIndicator

* Various fixes suggested by reviewer

* Fixed lint errors

* fix miscellaneous typos in dartdocs

* moar typos

* upgrade intl data; use local numerals in date picker

* use the shiny new public intl API

* address comments

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._

This adds an optional character counter and maxLength parameter to the TextField, as described in the Material Design Spec.

The counter text and style in the input decorator may be specified, but will default to the "right thing" if not specified, where the "right thing" is a counter that looks like "3 / 10" (if there are three characters entered into a field where maxLength is set to 10).

To limit the number of characters entered, I created a LengthLimitingTextFormatter that will limit the number of characters (Unicode runes) in the input, which can be used independently. The formatter is applied after the other formatters supplied (if any). Even if there is no decorator, the text field will limit the number of characters input if maxLength is set.

If maxLengthEnforced is set to false (it defaults to true), then the max length will not be enforced. In that case, if the text exceeds the length, then the counter will turn red, and it will make the divider turn red.

Revert "Revert "Fixes InputDecorator to work with textScaleFactor, fixes Material Design differences. (#12595)" (#12678)" (#12689)

This reverts commit 72dc7d91.

Re-landing my InputDecorator changes so that I can land the character counter PR.

Revert "Fixes InputDecorator to work with textScaleFactor, fixes Material Design differences. (#12595)" (#12678)

This reverts commit 67cf7918.

Reverting because this causes scuba regressions that I'd like to
address in another PR that is pending, but we'd like to roll Flutter.

https://github.com/flutter/flutter/issues/11992