Fix DropdownMenu can be focused and updated when disabled (#149737)

## Description

This PRs fixes `DropdownMenu` behaviors when disabled.

Before this PR the `DropdownMenu` value can be changed when `DropdownMenu.enabled` was false because the inner `IconButton` was not disabled so it can get focus (for instance using tab key).

After this PR, the inner `TextField` and the inner `IconButton` are disabled when `DropdownMenu.enabled` is false.

## Related Issue

Fixes https://github.com/flutter/flutter/issues/149598.

## Tests

Updates 1 test.
Adds 1 test.

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

@@ -655,7 +655,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
         style: effectiveStyle,
         leadingIcon: entry.leadingIcon,
         trailingIcon: entry.trailingIcon,
-        onPressed: entry.enabled
+        onPressed: entry.enabled && widget.enabled
           ? () {
               _localTextEditingController?.value = TextEditingValue(
                 text: entry.label,
@@ -676,7 +676,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
 
   void handleUpKeyInvoke(_) {
     setState(() {
-      if (!_menuHasEnabledItem || !_controller.isOpen) {
+      if (!widget.enabled || !_menuHasEnabledItem || !_controller.isOpen) {
         return;
       }
       _enableFilter = false;
@@ -695,7 +695,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
 
   void handleDownKeyInvoke(_) {
     setState(() {
-      if (!_menuHasEnabledItem || !_controller.isOpen) {
+      if (!widget.enabled || !_menuHasEnabledItem || !_controller.isOpen) {
         return;
       }
       _enableFilter = false;
@@ -786,7 +786,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
             isSelected: controller.isOpen,
             icon: widget.trailingIcon ?? const Icon(Icons.arrow_drop_down),
             selectedIcon: widget.selectedTrailingIcon ?? const Icon(Icons.arrow_drop_up),
-            onPressed: () {
+            onPressed: !widget.enabled ? null : () {
               handlePressed(controller);
             },
           ),
@@ -799,6 +799,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
 
         final Widget textField = TextField(
           key: _anchorKey,
+          enabled: widget.enabled,
           mouseCursor: effectiveMouseCursor,
           focusNode: widget.focusNode,
           canRequestFocus: canRequestFocus(),
@@ -825,7 +826,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
             }
             controller.close();
           },
-          onTap: () {
+          onTap: !widget.enabled ? null : () {
             handlePressed(controller);
           },
           onChanged: (String text) {
@@ -837,7 +838,6 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
           },
           inputFormatters: widget.inputFormatters,
           decoration: InputDecoration(
-            enabled: widget.enabled,
             label: widget.label,
             hintText: widget.hintText,
             helperText: widget.helperText,

packages/flutter/test/material/dropdown_menu_test.dart

@@ -83,11 +83,9 @@ void main() {
     expect(material.textStyle?.height, 1.43);
   });
 
-  testWidgets('DropdownMenu can be disabled', (WidgetTester tester) async {
-    final ThemeData themeData = ThemeData();
+  testWidgets('Inner TextField is disabled when DropdownMenu is disabled', (WidgetTester tester) async {
     await tester.pumpWidget(
       MaterialApp(
-        theme: themeData,
         home: Scaffold(
           body: SafeArea(
             child: DropdownMenu<TestMenu>(
@@ -100,7 +98,7 @@ void main() {
     );
 
     final TextField textField = tester.widget(find.byType(TextField));
-    expect(textField.decoration?.enabled, false);
+    expect(textField.enabled, false);
     final Finder menuMaterial = find.ancestor(
       of: find.byType(SingleChildScrollView),
       matching: find.byType(Material),
@@ -116,6 +114,25 @@ void main() {
     expect(updatedMenuMaterial, findsNothing);
   });
 
+  testWidgets('Inner IconButton is disabled when DropdownMenu is disabled', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/149598.
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: SafeArea(
+            child: DropdownMenu<TestMenu>(
+              enabled: false,
+              dropdownMenuEntries: menuChildren,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    final IconButton trailingButton = tester.widget(find.widgetWithIcon(IconButton, Icons.arrow_drop_down).first);
+    expect(trailingButton.onPressed, null);
+  });
+
   testWidgets('Material2 - The width of the text field should always be the same as the menu view',
     (WidgetTester tester) async {