Add `const Border.uniform()` (#30640)

`Border.all()` is a factory constructor and thus not const
constructible. This change adds a `const Border.uniform()`
constructor and makes `Border.all()` delegate to it. This allows
callers to more likely be able to make their widget tree const
constructible.

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

@@ -314,6 +314,16 @@ class Border extends BoxBorder {
        assert(bottom != null),
        assert(left != null);
 
+  /// Creates a border whose sides are all the same.
+  ///
+  /// The `side` argument must not be null.
+  const Border.uniform(BorderSide side)
+      : assert(side != null),
+        top = side,
+        right = side,
+        bottom = side,
+        left = side;
+
   /// A uniform border with all sides the same color and width.
   ///
   /// The sides default to black solid borders, one logical pixel wide.
@@ -323,7 +333,7 @@ class Border extends BoxBorder {
     BorderStyle style = BorderStyle.solid,
   }) {
     final BorderSide side = BorderSide(color: color, width: width, style: style);
-    return Border(top: side, right: side, bottom: side, left: side);
+    return Border.uniform(side);
   }
 
   /// Creates a [Border] that represents the addition of the two given

packages/flutter/test/painting/border_test.dart

@@ -13,6 +13,16 @@ void main() {
     expect(() => Border(bottom: nonconst(null)), throwsAssertionError);
   });
 
+  test('Border.uniform constructor', () {
+    expect(() => Border.uniform(null), throwsAssertionError);
+    const BorderSide side = BorderSide();
+    const Border border = Border.uniform(side);
+    expect(border.left, same(side));
+    expect(border.top, same(side));
+    expect(border.right, same(side));
+    expect(border.bottom, same(side));
+  });
+
   test('Border.merge', () {
     const BorderSide magenta3 = BorderSide(color: Color(0xFFFF00FF), width: 3.0);
     const BorderSide magenta6 = BorderSide(color: Color(0xFFFF00FF), width: 6.0);