This implements focus and hover handling for Material buttons. It inserts Focus widgets into the tree in order to allow buttons to be focusable via keyboard traversal (a.k.a. TAB traversal), and Listener widgets into the InkWell to allow the detection of hover states for widgets.

Addresses #11344, #1608, and #13264.

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

* Update flat_button.dart

* Added smaple code for Flat Button

* Update flat_button.dart

* Revert "Update flat_button.dart"

This reverts commit c8f00dff39e18b267295890bd0c5a86979b58c24.

* Revert "Added smaple code for Flat Button"

This reverts commit adc4234bc7956293a5b56f39701c6967fe3e919a.

* Update flat_button.dart

* Update flat_button.dart

* Update flat_button.dart

* remove white spaces

* remove white spaces

* add two different samples

* Update flat_button.dart

Previously, a disabled floating action button always had zero
elevation, which looks dumb.

This also fixes the issue whereby highlightElevation was not honoured
on floating action buttons.

This also fixes an issue I found during testing whereby setState was
being called during build when onHighlightChanged fired due to
onPressed becoming null while a gesture is ongoing (which triggers an
onTapCancel synchronously during build).

This reverts commit c689c97d.

The Dart analyzer in Google3 has been upgraded so we won't need this.

This reverts commit 80f80ab0.

For G3 Roll:

* Revert "MaterialButton must honor its minWidth and height parameters (#22919)"

This reverts commit a0233233.

* Revert "Update uses of ButtonTheme.bar: pass along the current Theme's colorScheme (#22827)"

This reverts commit 655bf6a2.

* Revert "ButtonTheme.of().colorScheme defers to Theme (#22880)"

This reverts commit a590940e.

* Revert "Bring TextTheme into alignment with the current Material spec (#22330)"

This reverts commit 8bfb4b3e.

* Revert "Added ColorScheme, updated ThemeData, ButtonTheme, material buttons (#22013)"

This reverts commit eea3465a.

* Manual adjustments to fix reversion issues.

We'll roll this back as soon as the Dart analyzer gets upgraded in
Google3.

Introduce an overall material "color scheme" and to make it possible to redefine how a component's visual properties bind to the color scheme and to the existing material Theme.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

This allows developers to control the clipBehavior of those buttons.

This reverts commit 989f5741.

This provides a way to draw colors, images, and general decorations on Material widgets, without interfering with InkWells that are further descendants of the widget.

This thus provides a cleaner way to solve the issue of FlatButtons and InkWells not working when placed over Image widgets than the old hack of introducing a transparency Material.

Fixes #3782.

Also, some fixes to documentation, and remove a redundant property on the Image widget.

* Implements FlatButton debugFillDescription

* Fix doc comment

* Override debugFillProperties instead of debugFillDescription

* Remove child debug property

This modifies the MaterialButton to expand vertically to fit the size of the contained child, no matter what the child widget is. It tries to be as close as possible to the minHeight constraint.

Also updated some doc comments to have headers (extra blank line after first sentence), and to wrap at 80 cols.

Addresses #12311

* more widget const constructors

* prefer const constructors

* address review comments

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

* address review comments

* adds highlight + splash color properties to material button

* add states to raised button + flat button

* fixes an issue where the custom highlight would not be properly set

* Add preliminary tests for theme highlight + splash and via theme

* documentation changes. tweaks per code review feedback

* comment out unused variable

* remove unused import

* documentation updates. removed commented code

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test

Fixes for: https://github.com/flutter/flutter/issues/7570 https://github.com/flutter/flutter/issues/7231 https://github.com/flutter/flutter/issues/2841 and others

The demo of the SimpleDialog had some useful code that should really be part of
the framework. This patch extracts it into a SimpleDialogOption widget.

Remove debugCheckHasScaffold because it is unused.

Also, add tests for InkWell, SimpleDialog, and other widgets.

Based on observations of usability study participant P1.

Fixes #3208.

We now use the `@required` annotation to encourage developers to
explicitly set onPressed and onChanged callbacks to null when that would
disable the widget.

Fixes #287

Fixes 3544

For consistency.

Fixes #4142

Making progress towards document all public APIs in material.dart. We're still
missing a few odds and ends (as well as missing docs in tabs.dart,
tooltip.dart, and two_level_list.dart).

Instead of incorporating the margin into the button, introduce a ButtonBar
widget that supplies the proper spacing between the buttons. Also, make these
buttons more configurable via ButtonTheme so that dialogs can change the
minWidth and padding of the buttons as required by the spec.

Fixes #1843
Fixes #3184

Also, clean up a few interfaces that looked awkward when writing docs.