v0.9.0-alpha+15

• Update dependencies for compatibility with Dart SDK v2.0.0-dev.62.0.
• Material Button: Remove styles for vertically aligned icon with text buttons.
• Material Checkbox: Add material-checkbox-color mixin and deprecate
  material-checkbox-theme.
• Material Datepicker: Use updated class names for ShowHideDirective.
• Material Expansion Panel: Add a mixin to show header actions on the bottom
  instead of on the left.
• Material Popup: Automatically restore focus on the popup source element when
  user tabs out.
• Material Progress: Add mixin for taller bars with rounded ends.
• Material Select:
  • Update to activedescendant ARIA pattern for a11y.
  • Ensure button text displays correctly when an error message shows below it.
• Material Slider:
  • Add disabled property.
  • Update size of click target.
• Material Radio:
  • Add role attribute.
  • Add material-radio-color mixin and deprecate material-radio-theme.
• Add DI modules to Laminate Overlay & Popup components.
• Update to SelectionModel.single(), SelectionModel.multi() etc. instead of
  deprecated SelectionModel.withList().
• Remove explicit Visibility.local from Angular components and directives, as
  this is the default now.
• Remove legacy CSS class names used by ShowHideDirective that conflict with
  Angular JS apps.
• Updates to conform to new Effective Dart guidelines.