We might want to let folks customize the inactive color too, but customizing
the active color is a good place to start.

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

Factor out a reusable interface called Decoration from BoxDecoration.

Make all the consumers of BoxDecoration and the erstwhile BoxPainter
into consumers of Decoration.

Make a BoxPainter be something you get from a Decoration, rather than
something to which you pass a BoxDecoration.

Rename Shape to BoxShape now that it's documented specifically as
applying to boxes.

Move EdgeDims to its own file.

Move FractionalOffset up so that it's with the other helper classes in
its file rather than alone at the end.

Minor change to RenderClipOval's hit testing to avoid taking an
unnecessary square root.

Rename BoxDecorationPosition to DecorationPosition since
RenderDecoratedBox now takes any Decoration.

Implement hit testing for rounded rects.

Rename AnimatedBoxDecorationValue to AnimatedDecorationValue, and make
it support lerping across any Decoration (by deferring to the objects
involved).

Instead of PointerInputEvent having a "type" field, we now have a
different class for each pointer type.

This has ripple effects throughout the system.

I also did code cleanup in affected files while I was there.

We can just use Canvas now.  The two are the same thing.

Also, refactor more common code into RenderToggleable and handle dark themes
more correctly.

Fixes #601

 - These controls now have proper radial reactions.
 - You can drag the switch.
 - The radio button animates properly.
 - There's a demo in the Material Gallery

Fixes #219

Shadows now render as three seprate MaskFilter.blur components per the most recent Material spec.

The shadows Map was replaced by a similar Map called elevationToShadow with entries that match the 10 elevations specifed by http://www.google.com/design/spec/what-is-material/elevation-shadows.html.

The "level" property (many classes) is now called "elevation", to match the Material spec.

BoxShadow now includes a spreadRadius parameter - as in CSS box-shadow. Renamed the BoxShadow blur property to blurRadius to further align BoxShadow with CSS box-shadow.

Make Radio widgets take a type that describes the type of their value,
so that you can catch when you use the wrong value.

Standardise on ValueChanged<Foo> instead of having a FooValueChanged
for every value of Foo.

This makes skyanalyzer also check the examples, and fixes everything it
found there.

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

Add type annotations in many places.
Fix some identifiers to have more lint-satisfying names.
Make all operator==s consistent in style.
Reorder some functions for consistency.
Make ParentData no longer dynamic, and fix all the code around that.

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

These are now part of material.dart.

All the use cases for EventDisposition are now addressed by the gesture
detection system.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.

We now expose idiomatic setters for these properties. Eventually we'll remove
the setter functions.

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.

The name `root` is confusing because this value isn't the root of anything.
It's just the associated `RenderObject` instance.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.

We still need to tune the various parameters, but this patch is a start.

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

This refactors Checkbox to own a RenderObject similar to how Switch was
refactored in https://github.com/domokit/sky_engine/pull/376 and
extracts common functionality for toggleable renderers into a base
class.  Switch and Checkbox's render objects derive from this base
class to add their own custom painting and theming logic.

This introduces the notion of event disposition and allows event
targets (widgets and render objects) to consume events that should not
be processed further. This is needed by the Switch component in the
Drawer in the stocks example. The Switch is embedded in a DrawerItem.
The Switch handles the gesture tap event to toggle its state and should
handle pointer events to allow swiping and draw its own radial
reaction. The DrawerItem also handles gesture taps to allow toggling
the switch value when tapping anywhere on the drawer and to draw its
own ink splash. When tapping on the switch, both the switch's render
object and the DrawerItem's listener are in the event dispatch path.
The Switch needs to signal in some fashion that it consumed the event
so the DrawerItem does not also try to toggle the switch's state.

This changes Switch from being a subclass of the Toggleable widget to
being a standalone Component with a custom RenderObject. This is to
enable adding Switch-specific radial reaction animations in a
subsequent patch. The CustomPaint logic that Toggleable was using does
not provide a nice way for a class and its subclass to both participate
in deciding to repaint.