Fixes #441

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.

Example:
  import 'package:sky/widgets.dart';
  void main() => runApp(new Center(child:
      new StatisticsOverlay.allEnabled()));