Handles keyboard input.
@param event
function keydown (event) {
    switch (event.keyCode) {
      case $mdConstant.KEY_CODE.DOWN_ARROW:
        if (ctrl.loading) return;
        event.stopPropagation();
        event.preventDefault();
        ctrl.index   = Math.min(ctrl.index + 1, ctrl.matches.length - 1);
        updateScroll();
        reportMessages(false, ReportType.Selected);
        break;
      case $mdConstant.KEY_CODE.UP_ARROW:
        if (ctrl.loading) return;
        event.stopPropagation();
        event.preventDefault();
        ctrl.index   = ctrl.index < 0 ? ctrl.matches.length - 1 : Math.max(0, ctrl.index - 1);
        updateScroll();
        reportMessages(false, ReportType.Selected);
        break;
      case $mdConstant.KEY_CODE.TAB:
        // If we hit tab, assume that we've left the list so it will close
        onListLeave();

        if (ctrl.hidden || ctrl.loading || ctrl.index < 0 || ctrl.matches.length < 1) return;
        select(ctrl.index);
        break;
      case $mdConstant.KEY_CODE.ENTER:
        if (ctrl.hidden || ctrl.loading || ctrl.index < 0 || ctrl.matches.length < 1) return;
        if (hasSelection()) return;
        event.stopPropagation();
        event.preventDefault();
        select(ctrl.index);
        break;
      case $mdConstant.KEY_CODE.ESCAPE:
        event.preventDefault(); // Prevent browser from always clearing input
        if (!shouldProcessEscape()) return;
        event.stopPropagation();

        clearSelectedItem();
        if ($scope.searchText && hasEscapeOption('clear')) {
          clearSearchText();
        }

        // Manually hide (needed for mdNotFound support)
        ctrl.hidden = true;

        if (hasEscapeOption('blur')) {
          // Force the component to blur if they hit escape
          doBlur(true);
        }

        break;
      default:
    }
  }