Add ExpansibleController.toggle method. (flutter#181320)

Closes flutter#181319

### Description

- Adds `ExpansibleController.toggle` method
- Adds test to verify that `ExpansibleController.toggle` works as
expected


## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.

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[Discord]:
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---------

Co-authored-by: Victor Sanni <victorsanniay@gmail.com>

Author: ksokolovskyi

packages/flutter/lib/src/widgets/expansible.dart

@@ -126,6 +126,21 @@ class ExpansibleController extends ChangeNotifier {
     _setExpansionState(false);
   }
 
+  /// Convenience method for toggling the current [isExpanded] status.
+  ///
+  /// Calling this method may cause the [Expansible] to rebuild, so it may not
+  /// be called from a build method.
+  ///
+  /// Calling this method will notify registered listeners of this controller
+  /// that the expansion state has changed.
+  ///
+  /// See also:
+  ///
+  ///  * [expand], which expands the [Expansible].
+  ///  * [collapse], which collapses the [Expansible].
+  ///  * [isExpanded] to check whether the [Expansible] is expanded.
+  void toggle() => isExpanded ? collapse() : expand();
+
   /// Finds the [ExpansibleController] for the closest [Expansible] instance
   /// that encloses the given context.
   ///

packages/flutter/test/widgets/expansible_test.dart

@@ -511,4 +511,34 @@ void main() {
     controller.expand();
     await tester.pump();
   });
+
+  testWidgets('Controller can be toggled', (WidgetTester tester) async {
+    final controller = ExpansibleController();
+    addTearDown(controller.dispose);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Expansible(
+          controller: controller,
+          bodyBuilder: (BuildContext context, Animation<double> animation) => const Text('Body'),
+          headerBuilder: (BuildContext context, Animation<double> animation) =>
+              GestureDetector(onTap: controller.toggle, child: const Text('Header')),
+        ),
+      ),
+    );
+
+    expect(find.text('Body'), findsNothing);
+    controller.toggle();
+    await tester.pumpAndSettle();
+    expect(find.text('Body'), findsOneWidget);
+
+    controller.toggle();
+    await tester.pumpAndSettle();
+    expect(find.text('Body'), findsNothing);
+
+    expect(find.text('Body'), findsNothing);
+    controller.toggle();
+    await tester.pumpAndSettle();
+    expect(find.text('Body'), findsOneWidget);
+  });
 }