As it stands, dropdowns currently do not focus the item that is selected, so if you select something on a dropdown, and then close it, and re-open it, then the new item is not auto-focused. This PR changes that so that selected value is focused by default when the dropdown is re-opened.

This re-lands the change that adds focus nodes, hover, and shortcuts to switches, checkboxes, and radio buttons. (#43213), with fixes for the web tests that weren't enabled in the master that it was synced to when I first landed it.

- Adds an assert message checking for exactly one unique value for `DropdownItem`s

* Fix DropdownButton disabledHint behavior

* Fix hint behavior when selectedItemBuilder is null

* Improve variable names, some formatting updates

* Create _DropdownMenuItemContainer widget

* Improve API docs to be consistent with hint/disabledHint actual behavior

This adds a Focus node to the DropdownButton widget, allowing it to receive keyboard focus, and to show a focus highlight. In addition, I added the ability to activate the dropdown using the "enter" key binding (which is bound to ActivateAction in the WidgetsApp).

Related Issues
Fixes #42646
Fixes #43008
Fixes #42511

This changes the DropDownButton so that instead of dismissing itself when any metrics change occurs, it only dismisses itself when the orientation changes.

This gets around the fact that we can't currently have a dropdown and a text field on the same page because the keyboard disappearing when the dropdown gets focus causes a metrics change, and the dropdown immediately disappears when activated.

It still will cause the keyboard to jump up and down between controls, but that's a larger issue. At least now we can use the two together again.

* Fix DropdownButton crash when hint and selectedItemBuilder are simultaneously defined

* Reprise: Dropdown Menu layout respects menu items intrinsic sizes

* updated per review feedback

* updated per review feedback

* Add dropdown button style doc example for unique text styles

This reverts commit e47a1dc2.

* Create 'center' template

* Implement DropdownButton.selectedItemBuilder

* Added properties in DropdownFormField to match DropdownButton

* Minor style guide related changes

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

Fixed an issue with a DropdownButton crashing when a style was used that didn't include a fontSize.

* fix some formatting issues

* address review comments

* fix indent

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

This converts all remaining "## Sample code" segments into snippets, and fixes
the snippet generator to handle multiple snippets in the same dartdoc block
properly.

I also generated, compiled, and ran each of the existing application samples,
and fixed them up to be more useful and/or just run without errors.

This PR fixes these problems with examples:

1. Switching tabs in a snippet now works if there is more than one snippet in
   a single dartdoc block.
2. Generation of snippet code now works if there is more than one snippet.
3. Contrast of text and links in the code sample block has been improved to
   recommended levels.
4. Added five new snippet templates, including a "freeform" template to make
   it possible to show examples that need to change the app instantiation.
5. Fixed several examples to run properly, a couple by adding the "Scaffold"
   widget to the template, a couple by just fixing their code.
6. Fixed visual look of some of the samples when they run by placing many
   samples inside of a Scaffold.
7. In order to make it easier to run locally, changed the sample analyzer to
   remove the contents of the supplied temp directory before running, since
   having files that hang around is problematic (only a problem when running
   locally with the `--temp` argument).
8. Added a `SampleCheckerException` class, and handle sample checking
   exceptions more gracefully.
9. Deprecated the old "## Sample code" designation, and added enforcement for
   the deprecation.
10. Removed unnecessary `new` from templates (although they never appeared in
   the samples thanks to dartfmt, but still).

Fixes #26398
Fixes #27411

* Adds DropDownFormField Convenience Class

If `items` or `onChanged` is null the button will be disabled, the down arrow will be grayed out, and
the new `disabledHint` will be shown (if provided).

* Adds comments clarifying the procedure used to render the menu as well as tests verifying various dropdown menu button positioning and initial scroll states.

In long lists this resulted in the dropdown scrolling to the very last
item in its list. Now clamping the value at `0.0`. Added a test to
verify that the selected item aligns with the button to test the offset.

Fixes flutter/flutter#15346

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Fix Dropdown where icon not set to end of button

* Expand DropdownButton based on isExpanded flag

* Add dropdown golden tests

Fixes #13135, #16606