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

* globalToLocal was just broken when there was a rotation and a
  translation at the same time. This fixes that and adds a test.

* update graphic used by spinning_mixed since the old one went 404.

* simplify some of the code in the demo.

* fix MatrixUtils.transformPoint to be consistent with how we transform
  points elsewhere.

* stop transforming points elsewhere, just use
  MatrixUtils.transformPoint.

* make the Widget binding handle not having a root element.

* make the spinning_mixed demo update its widget tree.

Having this base class lets classes like CustomPainter and DataTableSource be
more agnostic as to what's generating the repaints.

The new `empty_statements` lint (in the next DEV roll), flags these empty statements.  Harmless, except the one in `basic.dart` that fixes a real-life bug. :)

Also, make sure that the parent is notified when they change.

Fixes #2298

This introduces the key parts of a paginated data table, not including
the built-in pagination features.

* Provide more data for the data table demo, so there's data to page.

* Introduce a ChangeNotifier class which abstracts out
  addListener/removeListener/notifyListeners. We might be able to use
  this to simplify existing classes as well, though this patch doesn't
  do that.

* Introduce DataTableSource, a delegate for getting data for data
  tables. This will also be used by ScrollingDataTable in due course.

* Introduce PaginatedDataTable, a widget that wraps DataTable and only
  shows N rows at a time, fed by a DataTableSource.

It's safe to remove the unneeded `void`s from setters since the blocking issues in the
`always_declare_return_types` lint have been fixed (https://github.com/dart-lang/linter/).  We can also safely flip the bit on  `avoid_return_types_on_setters`.

A flow layout is optimized for reposition children with transformation
matrices. A flow layout can animate the position of its children very
efficiently.