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

* 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

* Added sample to AnimatedWidget

* Updated AnimatedWidget example to use listenable from parent

* Made progress private and changed widget name

Adds asserts on required constructor parameters to a transition widgets.
This improves debuggability for those who aren't developing with live
analysis. e.g. when `turns` is not specified in a RotationTransition
constructor:

  package:flutter/src/widgets/transitions.dart: Failed assertion: line 43 pos 15: 'listenable != null': is not true.

becomes:

  package:flutter/src/widgets/transitions.dart': Failed assertion: line 259 pos 15: 'turns != null': is not true.

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.

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

This adds the animation links for illustrations of the Transition widgets, as well as adding a cross-references to the other transition widgets in the "See also" for each of their docs.

* Visibility widget

This attempts to address the confusion around how to hide a widget subtree.

* Apply review comments

* More clarifications

Updated documentation of SizeTransition, since I found it less than intuitive.

I added some constructor asserts to make the (existing) contract more clear.

As pointed out in the bug, the issue is fixed by just clamping sizeFactor.value.

Fixes #16247 

For example, so that the gallery can override the media query globally.

* Make FadeTransition more efficient

* Update fade_transition_test.dart

* Update proxy_box.dart

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.

Fixes the remaining known issues with widgets supporting RTL.

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

Previously, we used `Alignment`, which was difficult to understand. Now,
we just use an `Offset` scaled to the child's size, which is much easier
to understand.

Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Fix a bunch of obvious RTL bugs from code inspection.

when describing non-nullable method arguments.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.