* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

Adds a VisualDensity.adaptivePlatformDensity static function that returns different values for visual density based on the defaultTargetPlatform. Returns compact for desktop platforms, and a default visual density for other platforms.

This converts Diagnosticable to be a mixin instead of an abstract class, so that it can be used to add diagnostics to classes which already have a base class.

It leaves in place the DiagnosticableMixin mixin, since there are some plugins that are still using it, and removing it would mean that those plugins wouldn't work with master branch Flutter anymore. DiagnosticableMixin will be removed once this mixin version of Diagnosticable makes its way to the stable branch.

* implicit-casts:false in flutter/lib/src/material

* address review comments

* Add a density attribute to ThemeData

* Simplify tests

* Review changes (Hans)

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

This PR adds TargetPlatform.macOS to the TargetPlatform enum. This allows us to begin implementation of some adaptive UI based on which target platform is desired.

I haven't updated the tests here, that will come in a follow-up PR.

* Updated the BottomAppBar to use elevation overlays when a dark theme is used.

* Moved the code for calculating the overlay color to a new utility
ElevationOverlay class and made it private by not including in the material package.

Also updated the documentation for the ThemeData constructor to include
descriptions of parameters that don't map to specific fields directly.

The Material Dark Theme spec has been updated (or soon will be) to clarify that the elevation overlay that is applied to surface colors to indicate elevation should be based off of the colorScheme.onSurface color and not white. This commit implements this change.

* set the copyWith toggleButtonsTheme argument type to ToggleButtonsThemeData

* add a ThemeData.copyWith test

* Added new ButtonBarTheme to replace the deprecated ButtonTheme.bar method.

* Responding to PR feedback.

* [Material] Create material Banner component (#36880)

This PR creates a new material widget for the Banner component. This includes a theme as well. This widget can be dropped into any application, ideally at the top of a listview or scrollview.

(cherry picked from commit 35b6d668)

Removed the use of ButtonTheme.bar in the Banner implementation.

* Updated documentation from PR review comments.

Provide a new ThemeData.from() method that constructs a Theme based off the colors in a given ColorScheme.

This PR creates a new material widget for the Banner component. This includes a theme as well. This widget can be dropped into any application, ideally at the top of a listview or scrollview.

* [Menu] Create Menu theme

* [Menu] Create Menu theme

* [Menu] Formatting changes for Menu theme

* [Menu] Fix spacing difference in theme_data.dart.

* [Menu] Fix spacing difference in theme_data.dart.

* Specifying types

* Formatting changes

* Address PR feedback

* Formatting changes

* Address PR feedback

* Add inherited widget

* Add inherited widget

* Address PR feedback and add inherited widget.

* Formatting changes.

* Address PR feedback

* Address PR feedback

* Address PR feedback

* Address PR feedback

* Introduces ToggleButtons widget

* Introduces ToggleButtonsTheme inherited widget

* Introduces ThemeData.toggleButtonsTheme property