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

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.

Extracted a superclass from AnimatedWidgetBaseState that is called ImplicitlyAnimatedWidgetState. ImplicitlyAnimatedWidgetState implicitly animates, but it does not call setState() automatically. Subclasses must respond to animation changes themselves. This is a useful optimization for Widgets whose render objects listen to animations directly. (#18173)

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.

For example, mention the icon used for the drawer menu in the docs (this helps people writing unit tests); add DefaultAssetBundle sample code.

* Added sample code to AnimatedOpacity docs

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.

We've added a bunch of non-animatable properties to DefaultTextStyle.
This adds them to AnimatedDefaultTextStyle so that you can use them
even when your text style is animating.