v0.9.0-alpha+8: Add Material Datepicker Components

• Add ShowHideDirective and GlobalEscapeDirective.
• Material Button: Update highlight styling.
• Material Chips: Allow overriding left-icon-color.
• Material ExpansionPanel:
  • Allow icon to be hidden and allow additional buttons in the header.
  • Increase mixin specificity.
• Material Input:
  • Add mixin for hiding underline.
  • Add mixins for trailing text color and padding.
• Material Menu:
  • Implement Focusable.
  • Add a flag to MenuItemGroup to explicitly specify a separator.
• Material Popup: Simplify positioning algorithm.
• Material Select:
  • Add mixin to allow for error text margin.
  • Refactor constructEncapsulatedCss to utils.
• Material Tab: Make text-wrap mixin more generic.
• Material Tree: Add a shouldExpandAllWhenFiltered input.
• Material Toggle: Fix aria-pressed state.
• Material Tooltip:
  • Allow paper tooltip CSS to be customizable.
  • Allow 'error_outline' as a valid icon.
  • Prevent paper tooltips from overflowing on small screens.
• Scorecard: Add stacking context.
• Delete deprecated LazyEventEmitter.
• Migrate uses of ElementRef to Element or HtmlElement.
• Cleanup type warnings and other Dart 2 fixes.
• Update documentation.