Add directional static members to AlignmentGeometry. (flutter#176571)

Closes flutter#176543

- Adds `topStart`, `topEnd`, `centerStart`, `centerEnd`, `bottomStart`,
and `bottomEnd` constants to `AlignmentGeometry` to enable
`AlignmentDirectional` quick usage with dot shorthand syntax

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we expect every widget to implement].
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- [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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---------

Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>

Author: ksokolovskyi

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

@@ -54,6 +54,32 @@ abstract class AlignmentGeometry {
   /// * [Alignment.topRight], which is the same thing.
   static const AlignmentGeometry topRight = Alignment.topRight;
 
+  /// The top corner on the "start" edge.
+  ///
+  /// {@template flutter.painting.alignment.directional.start}
+  /// This can be used to indicate an offset from the left in [TextDirection.ltr]
+  /// text and an offset from the right in [TextDirection.rtl] text without having
+  /// to be aware of the current text direction.
+  /// {@endtemplate}
+  ///
+  /// See also:
+  ///
+  /// * [AlignmentDirectional.topStart], which is the same thing.
+  static const AlignmentGeometry topStart = AlignmentDirectional.topStart;
+
+  /// The top corner on the "end" edge.
+  ///
+  /// {@template flutter.painting.alignment.directional.end}
+  /// This can be used to indicate an offset from the right in [TextDirection.ltr]
+  /// text and an offset from the left in [TextDirection.rtl] text without having
+  /// to be aware of the current text direction.
+  /// {@endtemplate}
+  ///
+  /// See also:
+  ///
+  /// * [AlignmentDirectional.topEnd], which is the same thing.
+  static const AlignmentGeometry topEnd = AlignmentDirectional.topEnd;
+
   /// The center point along the left edge.
   ///
   /// See also:
@@ -75,6 +101,24 @@ abstract class AlignmentGeometry {
   /// * [Alignment.centerRight], which is the same thing.
   static const AlignmentGeometry centerRight = Alignment.centerRight;
 
+  /// The center point along the "start" edge.
+  ///
+  /// {@macro flutter.painting.alignment.directional.start}
+  ///
+  /// See also:
+  ///
+  /// * [AlignmentDirectional.centerStart], which is the same thing.
+  static const AlignmentGeometry centerStart = AlignmentDirectional.centerStart;
+
+  /// The center point along the "end" edge.
+  ///
+  /// {@macro flutter.painting.alignment.directional.end}
+  ///
+  /// See also:
+  ///
+  /// * [AlignmentDirectional.centerEnd], which is the same thing.
+  static const AlignmentGeometry centerEnd = AlignmentDirectional.centerEnd;
+
   /// The bottom left corner.
   ///
   /// See also:
@@ -96,6 +140,24 @@ abstract class AlignmentGeometry {
   /// * [Alignment.bottomRight], which is the same thing.
   static const AlignmentGeometry bottomRight = Alignment.bottomRight;
 
+  /// The bottom corner on the "start" edge.
+  ///
+  /// {@macro flutter.painting.alignment.directional.start}
+  ///
+  /// See also:
+  ///
+  /// * [AlignmentDirectional.bottomStart], which is the same thing.
+  static const AlignmentGeometry bottomStart = AlignmentDirectional.bottomStart;
+
+  /// The bottom corner on the "end" edge.
+  ///
+  /// {@macro flutter.painting.alignment.directional.end}
+  ///
+  /// See also:
+  ///
+  /// * [AlignmentDirectional.bottomEnd], which is the same thing.
+  static const AlignmentGeometry bottomEnd = AlignmentDirectional.bottomEnd;
+
   double get _x;
 
   double get _start;

packages/flutter/test/painting/alignment_test.dart

@@ -344,12 +344,18 @@ void main() {
     expect(AlignmentGeometry.topLeft, Alignment.topLeft);
     expect(AlignmentGeometry.topCenter, Alignment.topCenter);
     expect(AlignmentGeometry.topRight, Alignment.topRight);
+    expect(AlignmentGeometry.topStart, AlignmentDirectional.topStart);
+    expect(AlignmentGeometry.topEnd, AlignmentDirectional.topEnd);
     expect(AlignmentGeometry.centerLeft, Alignment.centerLeft);
     expect(AlignmentGeometry.center, Alignment.center);
     expect(AlignmentGeometry.centerRight, Alignment.centerRight);
+    expect(AlignmentGeometry.centerStart, AlignmentDirectional.centerStart);
+    expect(AlignmentGeometry.centerEnd, AlignmentDirectional.centerEnd);
     expect(AlignmentGeometry.bottomLeft, Alignment.bottomLeft);
     expect(AlignmentGeometry.bottomCenter, Alignment.bottomCenter);
     expect(AlignmentGeometry.bottomRight, Alignment.bottomRight);
+    expect(AlignmentGeometry.bottomStart, AlignmentDirectional.bottomStart);
+    expect(AlignmentGeometry.bottomEnd, AlignmentDirectional.bottomEnd);
   });
 
   test('AlignmentDirectional.resolve with null TextDirection asserts', () {