* add missing trailing commas

* add more missing trailing commas

* add more and more missing trailing commas

* 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

* 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

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

This allows other code to get the same style without having to know exactly it is computed.

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.

Previously, ExpansionPanel would do weird things if interacted with
when it was already animating. This is fixed and there's now a test.

Also:

 * Minor fixes to make the gallery work in RTL, not that there's
   any way to see that without hard-coding the framework to RTL.
   But at least I'll be less annoyed when doing that.

 * Some trivial code and documentation cleanup.

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._

Fixes #11930

Add Diagnosticable base class and documentation

And add a test to verify we don't do this again.

Also fixes a small typo.

backgroundColor -> color
backgroundImage -> image
BackgroundImage -> DecorationImage

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

Turns out we have many immutable classes.

Fixes #6892

There are more places we can use this annotation, but this patch just gets us
started.

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

Nobody knew what a Block was.

Covers lib/ in package:flutter.

This patch aligns the iteration patterns used by animations and
ChangeNotifier. They now both respect re-entrant removal of listeners
and coalesce duplication registrations. (Also, ChangeNotifier
notification is no longer N^2).

This patch introduces ObserverList to avoid the performance regression that the
previous version of this patch caused.

Fixes #7533

Based on P5's experience

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

See #4434.

Changed MergableMaterial to animated between states that have dividers
and those that don't in order not to have dividers that appear
unanimatedly. Fixes #5847.

Fixes #5393.

This commit changes MergeableMaterial to include a flag that
specifies whether connected Material slices should have dividers
between them.

Because parent structure changes when slices gets separated and
merged, children widgets can be rebuilt redundantly. This commit
adds a global key to each child so that the framework always knows
its children apart.

MergeableMaterial is an animated container that knows how to merge
separate slices of Material together.