v0.9.0-beta+2

Breaking Changes

• Remove SelectableChangeNotifier and deprecate SelectableWithComposition
  with the intention to remove. They are widely unused, complicated the
  implementations, and unsound in Dart 2.

Component Updates

Material Dialog

• Fix bug where a disposable could be added to its disposer after it had already
  been destroyed.

Material Icon

• Add aria label and use them for trailing/leading icons of Material Input.

Material Popup

• Change <main> html tag to a <div>. HTML5 states there should only be one
  main tag per application.

Other Updates

Miscellaneous

• Add is{Selectable|Disabled|Hidden}In(), getOptionIn() and, filterWhere()
  as static helpers to Selectable.
• Add a lookup method for finding the closest Material Color name based on any
  input color.

Dart 2 Updates

• Fixes for Dart 2 compile-time and runtime errors.
• Applicaion of dartfmt --fix.