Fix MaterialDropdownSelect type error when clicking a deselect item.

PiperOrigin-RevId: 225102087

Author: creisman

angular_components/lib/material_select/shift_click_selection.dart

@@ -11,7 +11,11 @@ import 'package:angular_components/model/selection/selection_options.dart';
 
 /// Handles toggling selection of a range of options when shift is held down
 /// while clicking on the end value.
-abstract class ShiftClickSelectionMixin<T> implements ActivationHandler<T> {
+//
+// This class must support handling the deselect item as well as normal items,
+// so it cannot implement [ActivationHandler<T>].
+abstract class ShiftClickSelectionMixin<T>
+    implements ActivationHandler<dynamic /* T | String */ > {
   T _pivot;
 
   SelectionModel<T> get selection;
@@ -48,9 +52,11 @@ abstract class ShiftClickSelectionMixin<T> implements ActivationHandler<T> {
   /// If [event] is not a [MouseEvent] it will not be handled. This method
   /// does handle mouse events even if shift is not held down.
   @override
-  bool handle(UIEvent event, T activatedValue) {
+  bool handle(UIEvent event, dynamic activatedValue) {
     if (selection is! MultiSelectionModel || event is! MouseEvent) return false;
-    _handleClick(event, activatedValue);
+    // The deselect label is never shown with a MultiSelectionModel, so it's
+    // safe to assume activatedValue is of type T at this point.
+    _handleClick(event, activatedValue as T);
     return true;
   }
 }