[Material] Parent checkbox in DataTable should deselect all, if children checkboxes are disabled or selected  (#67938)

* Add tristate to parent checkbox for DataTable to follow Material specifications

* Add decoration parameter to DataTable and DataTableTheme

* migrate material to nullsafety

* address review comments

* address review comments

* fix build

This reverts commit cb4b8677.

* Fix sort indicator for DataTables

As per material spec ascending order should be shown via an upward arrow
This commit changes the displayed arrows accordingly.

* Test sort indicator orientation in DataTable

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

* replace simple Container(w & h) with SizedBox

* address review comments

* address review comments

* Change onSort and add tests

* Add doc

* Regression test

* Remove if (true)

* Make test clearer

* Clearer comment

This PR modifies the existing API docs samples to use DartPad so that all of the samples are now interactive apps on the API docs site.

It also removes the restriction for the max width of the description area so that the dartpad region can expand horizontally.

I updated the first paragraph on the API docs to indicate that Flutter is more than just mobile now (same text as the README.md at the top level).

I modified a few of the examples so that they looked nicer, and fit better on the page.

I added the sample description text above each DartPad instance, since that often defines the context of the example.

I removed animations and images when they were redundant with the sample content. There were a few that made sense to keep, so I did.

`showCheckboxColumn` will determine whether or not selectable rows in
DateTable or PaginatedDataTable will contain a leading CheckBox.

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

* address review comments

* Api Samples and Diagrams for the following classes and methods:

  - AlertDialog
  - SimpleDialog
  - SingleChildScrollView
  - showDatePicker
  - RichText
  - InputDecoration
  - Divider
  - StreamBuilder
  - DataTable
  - BottomSheet
    - showBottomSheet
    - showModalBottomSheet

* Update packages/flutter/lib/src/material/divider.dart
Co-Authored-By: Kate Lovett <katelovett@google.com>

* Update packages/flutter/lib/src/widgets/basic.dart
Co-Authored-By: Kate Lovett <katelovett@google.com>

* Update packages/flutter/lib/src/material/data_table.dart
Co-Authored-By: Kate Lovett <katelovett@google.com>

* Update packages/flutter/lib/src/material/bottom_sheet.dart
Co-Authored-By: Kate Lovett <katelovett@google.com>

* Update packages/flutter/lib/src/material/scaffold.dart
Co-Authored-By: Kate Lovett <katelovett@google.com>

* Move animation out of sample

* Remove "widget.size", replace with "60"

* 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

(Breaking Change) Move some hardcoded pixel values to reusable constants.

* Add Colors.white38

* Update ThemeData.disabledColor and ButtonThemeData.disabledColor to Colors.white38

* Update pre-existing tests to expect Colors.white38 instead of Colors.white30

* Update API documentation to reflect these changes

* Custom dataRowHeight for DataTable and PaginatedDataTable

* Custom headingRowHeight for DataTable and PaginatedDataTable

* Add optional horizontalMargin and columnSpacing properties to DataTable

* Add horizontalMargin and columnSpacing tests for DataTable and PaginatedDataTable

* use null aware operators

* rollback changes about null-aware operator

* disable lint prefer_is_not_empty