Fix dropdown crashing on list having duplicate items.

Author: BirjuVachhani

lib/src/transformers/node_transformers/passive_dropdown_transformer.dart

@@ -189,7 +189,7 @@ class PassiveDropdownWidget extends StatelessWidget {
           splashColor: node.properties.splashColor?.toFlutterColor(),
         ),
         child: DropdownButton<Object>(
-          value: items.isEmpty || !items.contains(value) ? null : value,
+          value: items.isEmpty || items.count(value) != 1 ? null : value,
           isDense: node.properties.dense,
           isExpanded: node.properties.expanded,
           autofocus: node.properties.autoFocus,

lib/src/utils/extensions.dart

@@ -1831,6 +1831,8 @@ extension IterableExt<V> on Iterable<V> {
   Map<K, V> mapBy<K>(K Function(V) key) => <K, V>{
         for (final value in this) key(value): value,
       };
+
+  int count(V item) => where((e) => e == item).length;
 }
 
 extension TabBarIndicatorSizeExtensions on TabBarIndicatorSizeEnum {