Add SizedBox.square() constructor (#84041)

* Add SizedBox.square() constructor

* Fix comment and move test case to the right file

* Add missing import statement

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

@@ -2250,6 +2250,12 @@ class SizedBox extends SingleChildRenderObjectWidget {
       height = size?.height,
       super(key: key, child: child);
 
+  /// Creates a box whose [width] and [height] are equal.
+  const SizedBox.square({Key? key, Widget? child, double? dimension})
+    : width = dimension,
+      height = dimension,
+      super(key: key, child: child);
+
   /// If non-null, requires the child to have exactly this width.
   final double? width;
 

packages/flutter/test/widgets/sized_box_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -190,4 +191,18 @@ void main() {
     );
     expect(patient.currentContext!.size, equals(Size.zero));
   });
+
+  testWidgets('SizedBox.square tests', (WidgetTester tester) async {
+    await tester.pumpWidget(
+        const SizedBox.square(
+          dimension: 100,
+          child: SizedBox.shrink(),
+        )
+    );
+
+    expect(
+      tester.renderObject<RenderConstrainedBox>(find.byType(SizedBox).first).additionalConstraints,
+      BoxConstraints.tight(const Size.square(100)),
+    );
+  });
 }