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.

* 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

* make BuildContext.{ancestorStateOfType,ancestorRenderObjectOfType,rootAncestorStateOfType} generic

* make BuildContext.inheritFromWidgetOfExactType generic

* make BuildContext.ancestorInheritedElementForWidgetOfExactType generic

* make BuildContext.ancestorWidgetOfExactType generic

* fix snippet

* bump scoped_model on temp version

* update names

* Revert "bump scoped_model on temp version"

This reverts commit d1fcbba028cdb07f44738d1652391692d1ea5ec0.

* address review comments

* fix ci

* address review comments

* repeat the deprecation notice

* fix uppercase

* use of recommanded deprecation syntax

* address review comment

* Fixed assets embedding

* ++

* Completed snippet audit

* Analyzer

This will fix a failure in dev/bots/analyze-sample-code.dart when using
the tip of tree Dart SDK.

This reverts commit 949023b2.

When submitting data to a server, callers need a callback that will
get invoked after all the individual form fields are saved. If they
have a button that submits the form, they could just do this logic
in the click handler for the button (save the form, then submit to
the server), but if they have more ways than one to submit the form
(i.e. hitting enter while in a text form field), then it becomes
more convoluted and calls for a unified callback that will get
notified when the form is submitted.

None of its callers were using the return value, so no need to
invoke the `isValid` getter unnecessarily.

To describe what its return value represents

Documentation that explains a common use case seen in Github issues where users were surprised by TextFormField growth to accommodate validation messages.

* Added code snippet for Form Class. ref:#21136

* Formatting adjustments.

* Changed wording related to GlobalKey.

* 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

Fix the spelling errors in the dartdocs for the framework.

There are no functionality changes here, just documentation fixes.

* Fix form field validate method being called if form field is disabled

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

fuchsia_tester.dart still assumes Dart 1. Previously, it ran tests directly
from source, flutter_platform.dart automatically runs a kernel compile when
operating in Dart 2 mode, but this assumes a functional Dart SDK is available
in the artifacts directly, and fuchsia_tester.dart mocks out the artifacts
directory with an empty temp dir.

Remaining work is:
1. Get the frontend server building as a dependency on Fuchsia.
2. Patch fuchsia_tester.dart to use a valid Dart SDK and frontend server.

This also reverts migration to Dart 2 typedef syntax.

This reverts commit 6c56bb24. (#18362)
This reverts commit 3daebd05. (#18316)

Now that Dart 1 is turned off, reapplying my change to turn on the prefer_generic_function_type_aliases analysis option, and fix all the typedefs to Dart 2 preferred syntax.

Also eliminated the unused analysis_options_repo.yaml file and turned on public_member_api_docs in analysys_options.yaml.

No logic changes, just changing the typedef syntax for all typedefs, and updating analysis options.

Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.

This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

This patch fixes a collection of issues with widgets involved in text
editing:

  * Fire widget.onChanged on EditableText value change:
    The value of an EditableText is composed of the text value as well
    as other editing-related data such as selection-related information.

    Previously, widget.onChanged() was only called for updates via
    updateEditingValue(). For pastes via a TextSelectionOverlay, updates
    are signalled via _handleSelectionOverlayChanged(), which only ever
    triggered widget.onSelectionChanged(), but not widget.onChanged().

    Both updateEditingValue() and _handleSelectionOverlayChanged()
    perform the value update via _formatAndSetValue(), which is where
    this patch moves the widget.onChanged() call.

  * Correctly update TextFormField value on edits via controller:
    The textual value of a TextFormField exists in two locations:
      1. FormField.value, as with all FormFields and subclasses.
      2. TextEditingController.value associated with the TextField
         underlying the TextFormField.

    Previously, edits to the TextEditingController associated with a
    TextFormField resulted in updates to the rendered TextField widget,
    but did not update TextFormField.value. FormField.value is updated
    via FormField's onChanged function, which is called from the
    EditableText underlying the TextField underlying the TextFormField.
    EditableText only fires onChanged when it receives changes from the
    engine. It does not fire onChanged for changes made to the
    underlying TextController, since the owner of the TextController is
    the one making these changes and thus, already aware of them.
    FormField, however, *does* need to listen to these changes to update
    its value.

  * Adds an initialValue parameter to the TextFormField constructor:
    FormField's constructor already takes an initialValue parameter,
    which specifies the initial value in the field, which is also the
    value to which reset() returns the field.

    Previously, TextFormField took its initial value from the controller
    value (if a controller was passed in) or the empty string (if not).
    This had the undesirable effect that calling reset() always resets
    the value to the current value of the controller... i.e., does
    nothing.

    We now take an initial value explicitly.

...and break the navigator<->routes circular dependency.

This is a step towards fixing https://github.com/flutter/flutter/issues/9577 but doesn't yet do so.

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

There are more places we can use this annotation, but this patch just gets us
started.

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Came across "can ve" while experimenting with Forms using VSCode and Dart Code. Glanced over the rest of the docs for typos while I was at it.