* Add a pageSnapping parameter to PageView

Setting the pageSnapping property allows extending the PageView scroll
behavior, such as custom scroll animations or custom scroll bars.

* Apply pageSnapping CR feedback

- Remove _kNonSnappingPhysics, use null instead.
- Minor code style fixes.
- It turns out that the forth state is Arkansas, not California.

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.

* Support accessibility labels and hints

* more tests

* ++

* review comments

* fix merge

* test fix

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.

* Without checking compatibility

* ++

* ++

* ++

* more docs

* merge with Hans; address comments

* merge material_localizations.dart

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

* Revert "Construct the accessibility channel's events by SemanticsEvent. (#12638)"

This reverts commit 4c1150dd.

* Revert "Revert "Add a way to get the furthest Navigator" (#12608)"

This reverts commit 79f13c36.

This reverts commit 964a138d.

It may (or may not) have caused a performance regression. Hard to say.

* Theme provides all TextStyle properties

* match field declaration order in the test

* Theme.of returns text style with inherit == false

* change TextStyle.inherit logic; docs

* add TextStyle.debugLabel

* address comments

* add debug labels to Typography text styles

This avoids some code duplication that existed before and will make
further modifications easier.

Also switched to relative paths for local packages.

* create auto back button behaviour

* cosmetic fidelity

* tests

* review

* document new icon

This refactoring allows us to have SemanticsEvent object for events that are not
associated with an accessibility node id.
And allow https://github.com/flutter/flutter/pull/12594 to be a bit
cleaner with a single place for accessibility channel documentation (the
SemanticsEvent classes documentation).

* Revert "BoxDecoration.borderRadius and RoundedRectangleBorder.borderRadius RTL (#12603)"

This reverts commit 07908916.

* Revert "Copy snapshot file into assets (#12523)"

This reverts commit f6135107.

* Revert "Add a rootNavigator option to Navigator.of (#12580)"

This reverts commit 822084b2.

* Revert "Fix error message to refer to actual ClipRect widget instead of non-existing RectClip (#12632)"

This reverts commit 1573d3b8.

* Revert "Roll engine to 5003703330f280325fa73080bb23b1ab7008b213 (#12629)"

This reverts commit 81a358b7.

* Make clean command top level

Fixes https://github.com/flutter/flutter/issues/10783

* remove backwards

The libimobiledevice suite of tools do not include version information.
A simple way to verify they meet our version requirements is to run
idevice_id -l, which will fail when older versions are run against iOS
devices with newer versions of iOS installed.

Unfortunately, idevice_id -l will also fail when libimobiledevice is up
to date, but the attached devices have never been paired with the host
machine in Xcode.

This patch updates the error message to help guide the user in such
situations.

Fixes InputDecorator to work with textScaleFactor, fixes Material Design differences.

There were a number of differences with the Material Design spec, including
several different padding values and underline thickness.  This corrects
that so that the decorator is in line with the Material Design spec now.

Also, the decorator properly handles changes to the textScaleFactor, where
before it would not re-layout when needed, painting the cursor and
underline incorrectly.

The decorator also now properly animates helper, error, and hint text when
the textScaleFactor or input decoration properties change.

Helper text is now properly displayed in dense mode, as the spec shows.
Before this change, it was never displayed in dense mode.

Fixes #12485

Hixie pointed out that what I had before was resetting all of the MediaQuery values, not just textScaleFactor. This should fix that.

Syncing source files is not needed when host sends kernel file to the device.