This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1482

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

Instead of trying to flush the detached children from the child list, we keep
the set of detached children up to date and query on every read.

We'll need this for RTL support because the RTL state will live in the widget
tree. Also, remove the `oldWidget` argument to updateRenderObject because there
aren't any clients for it.

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

This patch prepares us to pass a BuildContext to RenderObjectWidgets, which
will make it possible to rebuild RenderObjectElements:

 * Delay creation of the render object until mount(). That will let us pass
   `this` to createRenderObject and have the inherited elements be initialized.

 * Cleanup widgets that take builder closures to prepare for their
   RenderObjectElement to be rebuilt more often.

 * Add a test for the interaction between inherited widgets and
   MixedViewport.

Related to #2598

It's slightly more efficient.

Fixes #2085

This makes WidgetError more like RenderingError, which will aid with https://github.com/flutter/flutter/issues/2356.

Fixes https://github.com/flutter/flutter/issues/2443

Now we produce an error message that explains why this operation isn't
permitted by the framework.

Fixes #1925

The previous sample code isn't a common (or recommended) pattern.

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

Fixes #1695

This patch includes documentation for transitions.dart and for
scrollable.dart.

This patch changes the framework to walk the child list forwards so that build
functions with global side effects do sensible things. Specifically, if you
have a number of autofocusable children, the first one the list will acquire
the focus because it gets built first now.

Fixes #182

...instead of a deep walk.

It's confusing to have non-render-object widgets on either side of the
render object ones.

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

Make ancestorStateOfType() and ancestorRenderObjectOfType() support
subclassing.

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

- make them all have the same style
- make them all include the stack trace last
- make them all stop printing if their callback is set (if
  appropriate, they don't yet all have callbacks)

Fixes #1058.

Fixes https://github.com/flutter/flutter/issues/572

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

After this patch, if you try to use a widget that depends on being enclosed in
a material, you now get an assert and a debugPrint if you're not inside a
material.

Fixes #243

Fixes #689

The old name was confusing.

Fixes #462

- `Scaffold.of(context).showBottomSheet(widget);`
- Returns an object with .closed Future and .close() method.
- Uses a StateRoute to handle back button.
- Take the Navigator logic out of the BottomSheet widget.
- Support showing a sheet while an old one is going away.
- Add Navigator.remove().

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

When we reactivate a subtree that had a global key, we weren't updating the
parent data because:

1) The child wasn't in the tree when we updated the parent data element.
2) The activated child didn't go through mount (just through update).

This patch moves the parent data update work to when we attach the render
object, which we do both during mount and when reactivating a child.

Fixes #345