[flutter_tools] add the feature for single widget reloads (#61411)

Implements the flutter config feature for #61407 , but does not implement any of the functionality.

packages/flutter_tools/lib/src/features.dart

@@ -34,6 +34,9 @@ class FeatureFlags {
   /// Whether flutter desktop for Windows is enabled.
   bool get isWindowsEnabled => isEnabled(flutterWindowsDesktopFeature);
 
+  /// Whether fast single widget reloads are enabled.
+  bool get isSingleWidgetReloadEnabled => isEnabled(singleWidgetReload);
+
   /// Whether a particular feature is enabled for the current channel.
   ///
   /// Prefer using one of the specific getters above instead of this API.
@@ -65,6 +68,7 @@ const List<Feature> allFeatures = <Feature>[
   flutterLinuxDesktopFeature,
   flutterMacOSDesktopFeature,
   flutterWindowsDesktopFeature,
+  singleWidgetReload,
 ];
 
 /// The [Feature] for flutter web.
@@ -127,6 +131,28 @@ const Feature flutterWindowsDesktopFeature = Feature(
   ),
 );
 
+/// The fast hot reload feature for https://github.com/flutter/flutter/issues/61407.
+const Feature singleWidgetReload = Feature(
+  name: 'Hot reload optimization for changes to class body of a single widget',
+  configSetting: 'single-widget-reload-optimization',
+  master: FeatureChannelSetting(
+    available: true,
+    enabledByDefault: false,
+  ),
+  dev: FeatureChannelSetting(
+    available: true,
+    enabledByDefault: false,
+  ),
+  beta: FeatureChannelSetting(
+    available: true,
+    enabledByDefault: false,
+  ),
+  stable: FeatureChannelSetting(
+    available: true,
+    enabledByDefault: false,
+  ),
+);
+
 /// A [Feature] is a process for conditionally enabling tool features.
 ///
 /// All settings are optional, and if not provided will generally default to

packages/flutter_tools/test/integration.shard/command_output_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/features.dart';
 import 'package:flutter_tools/src/globals.dart' as globals;
 import 'package:process/process.dart';
 
@@ -55,6 +56,20 @@ void main() {
     expect(result.stdout, contains('Running shutdown hooks'));
   });
 
+  test('flutter config contains all features', () async {
+    final String flutterBin = globals.fs.path.join(getFlutterRoot(), 'bin', 'flutter');
+    final ProcessResult result = await const LocalProcessManager().run(<String>[
+      flutterBin,
+      'config',
+    ]);
+
+    // contains all of the experiments in features.dart
+    expect(result.stdout.split('\n'), containsAll(<Matcher>[
+      for (final Feature feature in allFeatures)
+        contains(feature.configSetting),
+    ]));
+  });
+
   test('flutter run --machine uses AppRunLogger', () async {
     final Directory directory = createResolvedTempDirectorySync('flutter_run_test.')
       .createTempSync('_flutter_run_test.')

packages/flutter_tools/test/src/testbed.dart

@@ -724,6 +724,7 @@ class TestFeatureFlags implements FeatureFlags {
     this.isMacOSEnabled = false,
     this.isWebEnabled = false,
     this.isWindowsEnabled = false,
+    this.isSingleWidgetReloadEnabled = false,
 });
 
   @override
@@ -738,6 +739,9 @@ class TestFeatureFlags implements FeatureFlags {
   @override
   final bool isWindowsEnabled;
 
+  @override
+  final bool isSingleWidgetReloadEnabled;
+
   @override
   bool isEnabled(Feature feature) {
     switch (feature) {