This slipped in with an earlier commit.

- 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

Turns out we weren't managing focus correct between navigator routes because we
were missing a Focus widget above the routes. However, adding this widget
caused us to explode at startup because the initial route was trying to move
focus during the build phase.

This patch teaches Focus to have an initiallyFocusedScope, which can be use to
initialize the child focus scope.

Fixes #4065

Also, add more dartdoc to widgets.dart.

Now MediaQuery.of always returns a non-null value. By default, you get the
values associated with the current ui.Window.

Fixes #2894

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

Making progress documenting the widget library.

When introducing Focus widgets for the Drawer (and ModalRoutes), we weren't
actually giving them the focus. Now we move the focus scope when pushing modal
routes.

Fixes #184

Fixes #1308

When opening the keyboard or focusing an input widget, we should scroll the
widget into view so that the user can see what they're typing.

Instead, require the developer to opt-in to autofocusing because autofocusing
can be disruptive.

Fixes #1307

Make ancestorStateOfType() and ancestorRenderObjectOfType() support
subclassing.

Fixes https://github.com/flutter/flutter/issues/1087

Require a Key on Input.

Simplify the API for Focus.at() and Focus.moveTo().
Fixes #236.
This will require an e-mail to flutter-dev.

Make Input grab focus onTap not onPointerDown.
Fixes #189.

Complain when you use Focus.at() with two different GlobalKeys that
are both in the tree at the same time.
Fixes #181.

Add dartdocs for Focus.moveTo() and Focus.moveScopeTo().

- Remove unused FocusChanged typedef.
- Remove unused centerOfAttentionHeroTag.
- Modernize static functions for interacting with Scrollable by moving them
  into the Scrollable class.

I'm planning on adding more .of() functions and so to avoid further
copypasta I'm providing some general utility functions here.

Turns out we were assuming that once the global key is gone, our
listener is unregistered. It isn't.

I'm planning on adding a test as part of my nested navigator work.

- Change RouteArguments to pass the route's BuildContext rather than
  the Navigator. This caused the bulk of the examples/ and .../test/
  changes (those are mostly mechanical changes). It also meant I could
  simplify Navigator.of().

- Make initState() actually get called when the State's Element is in
  the tree, so you can use Foo.of() functions there. Added a test for
  this also.

- Provide a RouteWidget so that routes have a position in the Widget
  tree. The bulk of the route logic is still in a longer-lived Route
  object for now.

- Make Route.setState() only rebuild the actual route, not the whole
  navigator.

- Provided a Route.of().

- Provided a Route.writeState / Route.readState API that tries to
  identify the clients by their runtimeType, their key, and their
  ancestors keys, up to the nearest ancestor with a GlobalKey.

- Made scrollables hook into this API to track state. Added a test to
  make sure this works.

- Fix the debug output of GestureDetector and the hashCode of
  MixedViewport.

- Fixed ScrollableWidgetListState<T> to handle infinite lists.

- truncate pixel values to 1dp since there's really no point being told
  the Size is 302.98732587287 by 648.28498579187.

- describe more Widgets so that debugDumpApp() is more useful.

- remove bufferValue from ProgressIndicator (cc @hansmuller) since it's
  not yet implemented.

- half-hearted toString() for ThemeData. There's no point making a
  complete one, since it would cause line-wrap even on big monitors in
  debugDumpApp dumps, and you can easily get the actual values from a
  debugging if that's the issue.

- flesh out BoxConstraints.toString() so that fully unconstrained and
  fully infinite constraints are called out explicitly. I experimented
  with adding even more special cases, e.g. calling out unconstrained
  widths with fixed heights, etc, but it made the output less readable.

- remove a redundant _updateVariable() in AnimatedContainer (cc
  @abarth).

- add more information to RenderView.toString().

These are now part of material.dart.

Focus.at() and company should be on Focus, not FocusState.

_notifyDescendants() was using the wrong runtimeType.

Let InheritedWidget update the descendants during build.

When you setState() during build, assert that you're not
markNeedsBuild()ing someone who isn't a descendant.

Typo in Widget.toString().

We need a short name more often than a tree dump, so toString() should
be the short name.

Make debugDumpRenderTree() a global like debugDumpApp(), for
consistency. It's hard to remember the
SkyBinding.instance.dumpRenderTree() incantation.

Fixes #1179.

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

This lets clients listen for when a widget with a given global key goes through
a sync operation. We'll need this for mimic to track its mimicable when it
moves around the tree.

It was confusing to have both widget.dart and widgets.dart

Scoped focus, so you can have a dialog with input controls and not lose your focus in the background.

This introduces a GlobalKey registry so that you can tell when a key
has gone away (so you can unfocus dead dialogs).

Also I added an assert that you're not calling setState() during a
build. It turns out that doing so means you have a bug, because since
you're dirty already (you're building), you won't get rebuilt.

The focus code itself is gnarly. It uses a Component and an internal
Inherited TagNode to manage the focus state, and does crazy things
like updating its state during build to pretend retroactively that it
was built with some other state, once someone asks for focus the first
time (i.e. the first time it's examined, so you can't tell that it was
in a different state before). It does this so that it can autofocus
controls which otherwise wouldn't be focused.

This moves all the focus management into Navigator and showDialog(),
so the complexity is all buried deep and not visible to apps,
hopefully.

To do something like two tabs that each have an Input widget that
needs to be focused when you switch panes, you'd need to have two
Focus objects, one in each tab, and you need to set their autofocus to
true (maybe that should be the default?).

Assert that there are no duplicates.
Export GlobalKey from basic.dart, so that people don't have to import widgets.dart just for that.
Fix the "initialFocus" feature which actually didn't work.

This patch provides a class to manage focus, Focus, and a class to
manage ownership of the keyboard, KeyboardHandle.

Inherited (in widgets.dart) is adjusted to support subclasses that
need to copy state from instance to instance.

A GlobalKey key type is introduced that is basically the same as
UniqueKey. Component classes that need a globally-unique key can
specify that their 'key' constructor argument is a GlobalKey.

Focus
-----

You can use `Focus.at(this)` to determine if you, a Component, are
currently focused.

You can use `Focus.moveTo(this)` to take the focus or give it to a
particular component.

For this to work, there has to be a Focus class in the widget
hierarchy.

Currently, there can only be one Focus class, because nested scopes
aren't supported. We should add support for that in a future patch.
See issue #229.

KeyboardHandle
--------------

Instead of directly interacting with the keyboard service, you now ask
for a KeyboardHandle using `_keyboard.show(client)`. This returns a
KeyboardHandle class. On that class, you can call `handle.release()`
when you want to hide the keyboard. If `handle.attached` is still
true, and you need to reshow the keyboard after the user hid it, then
you can can `handle.showByRequest()`.

The point of this is that the `keyboard.show()` method maintains the
invariant that only one KeyboardHandle is live at a time.

There are some issues with the keyboard service that I filed as a
result of doing this patch: #226 #227