Add `Border.symmetric` constructor (#47523)

packages/flutter/lib/src/painting/box_border.dart

@@ -326,6 +326,19 @@ class Border extends BoxBorder {
         bottom = side,
         left = side;
 
+  /// Creates a border with symmetrical vertical and horizontal sides.
+  ///
+  /// All arguments default to [BorderSide.none] and must not be null.
+  const Border.symmetric({
+    BorderSide vertical = BorderSide.none,
+    BorderSide horizontal = BorderSide.none,
+  }) : assert(vertical != null),
+       assert(horizontal != null),
+       left = horizontal,
+       top = vertical,
+       right = horizontal,
+       bottom = vertical;
+
   /// A uniform border with all sides the same color and width.
   ///
   /// The sides default to black solid borders, one logical pixel wide.

packages/flutter/test/painting/border_test.dart

@@ -23,6 +23,18 @@ void main() {
     expect(border.bottom, same(side));
   });
 
+  test('Border.symmetric constructor', () {
+    expect(() => Border.symmetric(vertical: nonconst(null)), throwsAssertionError);
+    expect(() => Border.symmetric(horizontal: nonconst(null)), throwsAssertionError);
+    const BorderSide side1 = BorderSide(color: Color(0xFFFFFFFF));
+    const BorderSide side2 = BorderSide(color: Color(0xFF000000));
+    const Border border = Border.symmetric(vertical: side1, horizontal: side2);
+    expect(border.left, same(side2));
+    expect(border.top, same(side1));
+    expect(border.right, same(side2));
+    expect(border.bottom, same(side1));
+  });
+
   test('Border.merge', () {
     const BorderSide magenta3 = BorderSide(color: Color(0xFFFF00FF), width: 3.0);
     const BorderSide magenta6 = BorderSide(color: Color(0xFFFF00FF), width: 6.0);