Add bySemanticsIdentifier finder for finding by identifier (#155571)

## Add `bySemanticsIdentifier` finder for finding by identifier

### Description

This pull request introduces a new finder, `CommonFinders.bySemanticsIdentifier`, to the Flutter testing framework. This finder allows developers to locate `Semantics` widgets based on their `identifier` property, enhancing the precision and flexibility of widget tests.

### Motivation

Establish a consistent and reliable method for locating elements in integration and end-to-end (e2e) tests. Unlike `label` or `key`, which may carry functional significance within the application, the `identifier` is purely declarative and does not impact functionality. Utilizing the `identifier` for finding semantics widgets ensures that tests can target specific elements without interfering with the app's behavior, thereby enhancing test reliability, maintainability, and reusability across testing frameworks.

### Changes

- **semantics.dart**
  - Updated documentation to mention that `identifier` can be matched using `CommonFinders.bySemanticsIdentifier`.
  
- **finders.dart**
  - Added the `bySemanticsIdentifier` method to `CommonFinders`.
  - Supports both exact string matches and regular expression patterns.
  - Includes error handling to ensure semantics are enabled during tests.
  
- **finders_test.dart**
  - Added tests to verify that `bySemanticsIdentifier` correctly finds widgets by exact identifier and regex patterns.
  - Ensures that the finder behaves as expected when semantics are not enabled.

### Usage

Developers can use the new finder in their tests as follows:

```dart
// Exact match
expect(find.bySemanticsIdentifier('Back'), findsOneWidget);

// Regular expression match
expect(find.bySemanticsIdentifier(RegExp(r'^item-')), findsNWidgets(2));
```

packages/flutter/lib/src/semantics/semantics.dart

@@ -1213,7 +1213,8 @@ class SemanticsProperties extends DiagnosticableTree {
   /// This value is not exposed to the users of the app.
   ///
   /// It's usually used for UI testing with tools that work by querying the
-  /// native accessibility, like UIAutomator, XCUITest, or Appium.
+  /// native accessibility, like UIAutomator, XCUITest, or Appium. It can be
+  /// matched with [CommonFinders.bySemanticsIdentifier].
   ///
   /// On Android, this is used for `AccessibilityNodeInfo.setViewIdResourceName`.
   /// It'll be appear in accessibility hierarchy as `resource-id`.

packages/flutter_test/lib/src/finders.dart

@@ -470,8 +470,8 @@ class CommonFinders {
 
   /// Finds a standard "back" button.
   ///
-  /// A common element on many user interfaces is the "back" button. This is the
-  /// button which takes the user back to the previous page/screen/state.
+  /// A common element on many user interfaces is the "back" button. This is
+  /// the button which takes the user back to the previous page/screen/state.
   ///
   /// It is useful in tests to be able to find these buttons, both for tapping
   /// them or verifying their existence, but because different platforms and
@@ -555,11 +555,49 @@ class CommonFinders {
   ///
   /// If the `skipOffstage` argument is true (the default), then this skips
   /// nodes that are [Offstage] or that are from inactive [Route]s.
-  Finder bySemanticsLabel(Pattern label, { bool skipOffstage = true }) {
+  Finder bySemanticsLabel(Pattern label, {bool skipOffstage = true}) {
+    return _bySemanticsProperty(
+      label,
+      (SemanticsNode? semantics) => semantics?.label,
+      skipOffstage: skipOffstage,
+    );
+  }
+
+  /// Finds [Semantics] widgets matching the given `identifier`, either by
+  /// [RegExp.hasMatch] or string equality.
+  ///
+  /// This allows matching against the identifier of a [Semantics] widget, which
+  /// is a unique identifier for the widget in the semantics tree. This is
+  /// exposed to offer a unified way widget tests and e2e tests can match
+  /// against a [Semantics] widget.
+  ///
+  /// ## Sample code
+  ///
+  /// ```dart
+  /// expect(find.bySemanticsIdentifier('Back'), findsOneWidget);
+  /// ```
+  ///
+  /// If the `skipOffstage` argument is true (the default), then this skips
+  /// nodes that are [Offstage] or that are from inactive [Route]s.
+  Finder bySemanticsIdentifier(Pattern identifier, {bool skipOffstage = true}) {
+    return _bySemanticsProperty(
+      identifier,
+      (SemanticsNode? semantics) => semantics?.identifier,
+      skipOffstage: skipOffstage,
+    );
+  }
+
+  Finder _bySemanticsProperty(
+    Pattern pattern,
+    String? Function(SemanticsNode?) propertyGetter,
+    {bool skipOffstage = true}
+  ) {
     if (!SemanticsBinding.instance.semanticsEnabled) {
-      throw StateError('Semantics are not enabled. '
-                       'Make sure to call tester.ensureSemantics() before using '
-                       'this finder, and call dispose on its return value after.');
+      throw StateError(
+        'Semantics are not enabled. '
+        'Make sure to call tester.ensureSemantics() before using '
+        'this finder, and call dispose on its return value after.',
+      );
     }
     return byElementPredicate(
       (Element element) {
@@ -568,13 +606,13 @@ class CommonFinders {
         if (element is! RenderObjectElement) {
           return false;
         }
-        final String? semanticsLabel = element.renderObject.debugSemantics?.label;
-        if (semanticsLabel == null) {
+        final String? propertyValue = propertyGetter(element.renderObject.debugSemantics);
+        if (propertyValue == null) {
           return false;
         }
-        return label is RegExp
-            ? label.hasMatch(semanticsLabel)
-            : label == semanticsLabel;
+        return pattern is RegExp
+            ? pattern.hasMatch(propertyValue)
+            : pattern == propertyValue;
       },
       skipOffstage: skipOffstage,
     );

packages/flutter_test/test/finders_test.dart

@@ -299,6 +299,55 @@ void main() {
       expect(find.bySemanticsLabel('Foo'), findsOneWidget);
       semanticsHandle.dispose();
     });
+
+    testWidgets('Throws StateError if semantics are not enabled (bySemanticsIdentifier)', (WidgetTester tester) async {
+      expect(
+        () => find.bySemanticsIdentifier('Add'),
+        throwsA(
+          isA<StateError>().having(
+            (StateError e) => e.message,
+            'message',
+            contains('Semantics are not enabled'),
+          ),
+        ),
+      );
+    }, semanticsEnabled: false);
+
+    testWidgets('finds Semantically labeled widgets by identifier', (WidgetTester tester) async {
+      final SemanticsHandle semanticsHandle = tester.ensureSemantics();
+      await tester.pumpWidget(_boilerplate(
+        Semantics(
+          identifier: 'Add',
+          button: true,
+          child: const TextButton(
+            onPressed: null,
+            child: Text('+'),
+          ),
+        ),
+      ));
+      expect(find.bySemanticsIdentifier('Add'), findsOneWidget);
+      semanticsHandle.dispose();
+    });
+
+    testWidgets('finds Semantically labeled widgets by identifier RegExp', (WidgetTester tester) async {
+      final SemanticsHandle semanticsHandle = tester.ensureSemantics();
+      // list of elements with a prefixed identifier
+      await tester.pumpWidget(_boilerplate(
+        Row(children: <Widget>[
+          Semantics(
+            identifier: 'item-1',
+            child: const Text('Item 1'),
+          ),
+          Semantics(
+            identifier: 'item-2',
+            child: const Text('Item 2'),
+          ),
+        ]),
+      ));
+      expect(find.bySemanticsIdentifier('item'), findsNothing);
+      expect(find.bySemanticsIdentifier(RegExp(r'^item-')), findsNWidgets(2));
+      semanticsHandle.dispose();
+    });
   });
 
   group('byTooltip', () {