* migrate animation to nullsafety

* nullable tween

* fix generic type issue

* address review comment

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

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.

This adds CatmullRomCurve animation curve, and a CatmullRomSpline, which is what it uses to do interpolation.

This allows us to create animation curves which can smoothly interpolate the values given to the curve.

Since I've introduced a 2D spline curve, I also created a Curve2D base class for such parametric curves.

* 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

* Add curve fastInFastOut

* Rename it to slowCenter

* Rename to slowMiddle

* Add continuous test

Re-applies #27409 by reverting #27919

The analyzer issue (dart-lang/sdk#35940) has been temporarily
worked around by virtue of #27929

This makes the code more consistent but also fixes our flutter-analyze bot.

Issue https://github.com/dart-lang/sdk/issues/35940

This reverts commit d8cd2ff4.

* Changes Cupertino page transition curves to make paging animations more similar to those of native iOS

* Add Robert Penner’s easing functions

These easing functions are the most popular functions in use on the web
and other platforms. Including them in Flutter will encourage users to
create more engaging and attractive animations.

* fix Curves.bounceInOut

* assert maximum slope

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Enables animated curve diagrams in the documentation, replacing the static images.

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

Our style guide says the k's are not necessary, and it seems like a good idea to make all the code be consistent on this.

Only naming changes to private vars: no logic changes.

I'm moving the assets in the assets-for-api-docs repo to a slightly different location to help with organization in that repo, so this PR points the doc URLs to the new location. The old assets won't be removed until this PR makes its way to the API docs website.

No documentation or code changes here, other than changing doc image URLs.