This reverts commit b39e315e.

Breaking change to extremely rarely used ParentDataWidget.debugDescribeInvalidAncestorChain api changing the return type of the method from String to DiagnosticsNode.

This reverts commit d92b3b40.

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

* Re-enable const

* fix indentof statements in BlockFunctionBody

* fix indentof statements in BlockFunctionBody in tests

* Add curve fastInFastOut

* Rename it to slowCenter

* Rename to slowMiddle

* Add continuous test

* 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

Re-applies #27409 by reverting #27919

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

This reverts commit d8cd2ff4.

* 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

* Future<void> main

* Future<void>.delayed

* prefer_void_to_Null

* address review comments

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

Make it possible to define an animation by stringing together a sequence of tweens.

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.