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

* address review comments

* address review comment

* 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

* Minor improvements to framework documentation.

* maybePop documentation fix

* Mark some flutter_test arguments required.

Without these set, we get a crash.

* Minor correction to ListView docs

I noticed that we were pretty inconsistent with the way that we checked the value of clipBehavior in the framework, so I normalized the usages and updated docs where necessary.

This is a breaking change if you used to pass null explicitly to FlatButton, OutlineButton or RaisedButton constructors, expecting to get Clip.none. It will now assert if you do that. Existing implementations that pass null implicitly by not specifying clipBehavior won't need to change their call sites. It always implicitly defaulted to Clip.none before, and it will continue to do that, it's only places where it was explicitly set to null in order to get the implicit default that it will fail.

TweenAnimationBuilder for building custom animations without managing an AnimationController (#38317)

When I added reverseDuration to animation controllers in #32730, I also added it to the implicit animations that Flutter has. However, as @efortuna pointed out to me, it doesn't actually do anything there, since all of the intrinsic animations run forwards, not in the reverse direction, and there's no way to reverse them.

So, this PR removes the reverseDuration argument from the implicit animations to avoid confusion.

Fixes #35769

* add sample code for AnimatedContainer

* use stateful_widget_scaffold snippet template

* add sample explanation

* add setState() to example assumptions for analyzer

* update description

* use snippet instead of sample

* change sample to match assets-for-api-docs

* remove constant

* update AnimatedContainer sample description

add indication the example is depicted by the video and animates when
tapped

* fix example formatting

* make sample conform to 80 col limit

* add "implemented below" to illustration description

* formatting for readability

This is a follow-on to #33370 based on review comments
therein.

This adds a reverseDuration parameter to AnimationController so that the animation has a different duration when going in reverse as it does going forward.

The current documentation for `AnimatedWidgetBaseState.forEachTween` is exhaustive but a little hard to grok. This dartdoc example provides some clarity.

* 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 existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

This adds links to the already existing animated diagrams in the asset-for-api-docs repo.

This reverts commit 8de0e15e and
updates the goldens versoin.

This requires https://github.com/flutter/flutter/pull/20751 to
land first.

This fixes https://github.com/flutter/flutter/issues/18057

See details in our proposal for this breaking API change and #18057. This PR setup all code paths to allow the change but doesn't change the clip behavior by itself. We'll change `defaultClipBehavior` from `Clip.antiAlias` to `Clip.none` in the following PR to change the clip behavior and update tests.

Fixes #19617

The verbosity change is to help track down timeouts that currently look like:

```
C:\Windows\Temp\flutter sdk>call bin\flutter.bat config --no-analytics
Checking Dart SDK version...
Downloading Dart SDK from Flutter engine c5a63d28bf3735569c8187753bc490d8351a8363...
Unzipping Dart SDK...
Updating flutter tool...
```

* Use RenderAnimatedOpacity within AnimatedOpacity widget (#15466)

* Fixed minor bug in RenderAnimatedOpacity

* Updated protected API for ImplicitlyAnimatedWidget