Wire selectedItemBuilder through DropdownButtonFormField (#44160)

packages/flutter/lib/src/material/dropdown.dart

diff --git a/packages/flutter/lib/src/material/dropdown.dart b/packages/flutter/lib/src/material/dropdown.dart
index b542631..2106e6f 100644
--- a/packages/flutter/lib/src/material/dropdown.dart
+++ b/packages/flutter/lib/src/material/dropdown.dart
@@ -1299,6 +1299,7 @@
     Key key,
     T value,
     @required List<DropdownMenuItem<T>> items,
+    DropdownButtonBuilder selectedItemBuilder,
     Widget hint,
     @required this.onChanged,
     this.decoration = const InputDecoration(),
@@ -1347,6 +1348,7 @@
                child: DropdownButton<T>(
                  value: value,
                  items: items,
+                 selectedItemBuilder: selectedItemBuilder,
                  hint: hint,
                  onChanged: onChanged == null ? null : field.didChange,
                  disabledHint: disabledHint,

packages/flutter/test/material/dropdown_form_field_test.dart

diff --git a/packages/flutter/test/material/dropdown_form_field_test.dart b/packages/flutter/test/material/dropdown_form_field_test.dart
index c1baab0..4000e17 100644
--- a/packages/flutter/test/material/dropdown_form_field_test.dart
+++ b/packages/flutter/test/material/dropdown_form_field_test.dart
@@ -586,4 +586,48 @@
       );
     }
   });
+
+  testWidgets('DropdownButtonFormField - selectedItemBuilder builds custom buttons', (WidgetTester tester) async {
+    const List<String> items = <String>[
+      'One',
+      'Two',
+      'Three',
+    ];
+    String selectedItem = items[0];
+
+    await tester.pumpWidget(
+      StatefulBuilder(
+        builder: (BuildContext context, StateSetter setState) {
+          return MaterialApp(
+            home: Scaffold(
+              body: DropdownButtonFormField<String>(
+                value: selectedItem,
+                onChanged: (String string) => setState(() => selectedItem = string),
+                selectedItemBuilder: (BuildContext context) {
+                  int index = 0;
+                  return items.map((String string) {
+                    index += 1;
+                    return Text('$string as an Arabic numeral: $index');
+                  }).toList();
+                },
+                items: items.map((String string) {
+                  return DropdownMenuItem<String>(
+                    child: Text(string),
+                    value: string,
+                  );
+                }).toList(),
+              ),
+            ),
+          );
+        },
+      ),
+    );
+
+    expect(find.text('One as an Arabic numeral: 1'), findsOneWidget);
+    await tester.tap(find.text('One as an Arabic numeral: 1'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('Two'));
+    await tester.pumpAndSettle();
+    expect(find.text('Two as an Arabic numeral: 2'), findsOneWidget);
+  });
 }
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