FormField is now a widget that can contain any type of field. Input no
longer has special code to handle form fields. Instead, there is a
helper widget InputFormField for using an Input inside a FormField.

Fixes https://github.com/flutter/flutter/issues/6097 and based on
feedback from the same.

Fixes #6591

Developers need to get the size of the BuildContext sufficiently often
that we should provide a convenient getter for the value. Having this
getter is also an opportunity to catch common mistakes and provide
useful error messages that guide developers towards better patterns.

Fixes #2321

Fixes #6209

This patch adds `@checked` everywhere is needed to remove the
`strong_mode_invalid_method_override` strong mode error.

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.

This silences an assertion that fired when reparenting a widget with a
global key inside a LayoutBuilder callback when that callback also
happened to call setState (directly or indirectly) on that widget.

Normally such setStates are considered ok since we know we haven't
cleaned that subtree yet, but we were not correctly handling the case
where the list needed resorting in that situation.

The debugging logic was lagging behind the recent actual logic changes,
especially around buildScope.

Previously in some rather esoteric cases involving global keys reversing
relative positions we would forget to build everyone.

There are still more to go, but this patch completes the widget section.

See #4434.

Also, make ProxyWidget and ProxyElement public.

Fixes #4505

Fixes https://github.com/flutter/flutter/issues/5840

This allows us to simplify the logic around inherited widgets e.g. in
the Image widget.

I forgot that it was possible for the root view to get marked dirty
without getting a new widget. This fixes that case to work.

I did a pass through some of the code cleaning minor things up.

Previously, pumpWidget() would do a partial pump (it didn't trigger
Ticker callbacks or post-frame callbacks), and pump() would do a full
pump. This patch brings them closer together. It also makes runApp run a
full actual frame, rather than skipping the transient callback part of
the frame logic. Having "half-frames" in the system was confusing and
could lead to bugs where code expecting to run before the next layout
pass didn't because a "half-frame" ran first.

Also, make Tickers start ticking in the frame that they were started in,
if they were started during a frame. This means we no longer spin a
frame for t=0, we jump straight to the first actual frame.

Other changes in this patch:

* rename WidgetsBinding._runApp to WidgetsBinding.attachRootWidget, so
  that tests can use it to more accurately mock out runApp.

* allow loadStructuredData to return synchronously.

* make handleBeginFrame handle not being given a time stamp.

* make DataPipeImageProvider.loadAsync protected (rather than private),
  and document it. There wasn't really a reason for it to be private.

* fix ImageConfiguration.toString.

* introduce debugPrintBuildScope and debugPrintScheduleBuildForStacks,
  which can help debug problems with widgets getting marked as dirty but
  not cleaned.

* make debugPrintRebuildDirtyWidgets say "Building" the first time and
  "Rebuilding" the second, to make it clearer when a widget is first
  created. This makes debugging widget lifecycle issues much easier.

* make debugDumpApp more resilient.

* debugPrintStack now takes a label that is printed before the stack.

* improve the banner shown for debugPrintBeginFrameBanner.

* various and sundry documentation fixes

Previously, if a StatefulWidget was marked dirty, then removed from the
build, then reinserted using the exact same widget under a widget under
a LayoutBuilder, it wouldn't rebuild.

This fixes that.

It also introduces an assert that's supposed to catch SizeObserver-like
behaviour. Rather than make this patch even bigger, I papered over two
pre-existing bugs which this assert uncovered (and fixed the other
problems it found):

   https://github.com/flutter/flutter/issues/5751
   https://github.com/flutter/flutter/issues/5749

We should fix those before 1.0 though.

Fixes https://github.com/flutter/flutter/issues/5283

Other changes in this patch:

Rename OffStage to Offstage.
Fixes https://github.com/flutter/flutter/issues/5378

Add a lot of docs.

Some minor punctuation and whitespace fixes.

Somehow we missed this before.

Fixes https://github.com/flutter/flutter/issues/5615

Fixes https://github.com/flutter/flutter/issues/5588

...so that you can use hot reload mode to update assets.

And make Scrollbar work with LazyBlock.

And an about box to the Stocks sample app.

- show the next error fully after a hot reload
- hide _AssertionError in stacks
- immediately rebuild after a reassemble, so that hit tests work
- catch errors when notifying global key listeners

Also, add a few more debugging flags.

Fixes #4761

Now that protected can be accessed from inside the same library, we can
use protected in a number of new places and we can remove some
trampolines we were using to work around its previous semantics.

Bye-bye `@protected `trampolines! :)

This patch documents State in detail because it's a very common class
for developers to work with and it has a number of subtles.