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.

Adds a default title to the Title widget, fixes #6802.

Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

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

* preview of prefer_asserts_in_initializer_list lint

* fix issue

Instead, we now interact with the system navigator via SystemNavigator.

* System chrome overlay style fixes

1) Add a cache to avoid superfluous calls to the embedder
2) Coalesce calls to the embedder that are made in the same event loop
3) Move call site to material's AppBar from Title widget

#4461

Fixes 3544

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

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

Turns out Android crashes if the colour is not opaque, so we enforce
that at the Dart level.

Also, since label and colour are both actually optional, make them
named arguments.

- 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.

What's important about this code is that it's presenting services outside the
VM, not the particular technology used to present the services.

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.