Add convenience static members to AlignmentGeometry (#169709)

This adds convenience members to `AlignmentGeometry` in preparation for
Dart shorthands.

Before:

```dart
Container(
  alignment: Alignment.topLeft,
  child: Text('Hello world'),
),
```

After (with Dart shorthands):

```dart
Container(
  alignment: .topLeft,
  child: Text('Hello world'),
),
```

Prior art: https://github.com/flutter/flutter/pull/165597

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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].
- [ ] 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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on [Discord].

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

@@ -15,8 +15,8 @@ import 'basic_types.dart';
 /// Base class for [Alignment] that allows for text-direction aware
 /// resolution.
 ///
-/// A property or argument of this type accepts classes created either with [
-/// Alignment] and its variants, or [AlignmentDirectional.new].
+/// A property or argument of this type accepts classes created either with
+/// [Alignment] and its variants, or [AlignmentDirectional.new].
 ///
 /// To convert an [AlignmentGeometry] object of indeterminate type into an
 /// [Alignment] object, call the [resolve] method.
@@ -32,6 +32,69 @@ abstract class AlignmentGeometry {
   /// Creates a directional alignment, or [AlignmentDirectional].
   const factory AlignmentGeometry.directional(double start, double y) = AlignmentDirectional;
 
+  /// The top left corner.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.topLeft], which is the same thing.
+  static const AlignmentGeometry topLeft = Alignment.topLeft;
+
+  /// The center point along the top edge.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.topCenter], which is the same thing.
+  static const AlignmentGeometry topCenter = Alignment.topCenter;
+
+  /// The top right corner.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.topRight], which is the same thing.
+  static const AlignmentGeometry topRight = Alignment.topRight;
+
+  /// The center point along the left edge.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.centerLeft], which is the same thing.
+  static const AlignmentGeometry centerLeft = Alignment.centerLeft;
+
+  /// The center point, both horizontally and vertically.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.center], which is the same thing.
+  static const AlignmentGeometry center = Alignment.center;
+
+  /// The center point along the right edge.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.centerRight], which is the same thing.
+  static const AlignmentGeometry centerRight = Alignment.centerRight;
+
+  /// The bottom left corner.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.bottomLeft], which is the same thing.
+  static const AlignmentGeometry bottomLeft = Alignment.bottomLeft;
+
+  /// The center point along the bottom edge.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.bottomCenter], which is the same thing.
+  static const AlignmentGeometry bottomCenter = Alignment.bottomCenter;
+
+  /// The bottom right corner.
+  ///
+  /// See also:
+  ///
+  /// * [Alignment.bottomRight], which is the same thing.
+  static const AlignmentGeometry bottomRight = Alignment.bottomRight;
+
   double get _x;
 
   double get _start;

packages/flutter/test/painting/alignment_test.dart

@@ -338,4 +338,16 @@ void main() {
     expect(const AlignmentGeometry.xy(4, 5), const Alignment(4, 5));
     expect(const AlignmentGeometry.directional(4, 5), const AlignmentDirectional(4, 5));
   });
+
+  test('AlignmentGeometry static members', () {
+    expect(AlignmentGeometry.topLeft, Alignment.topLeft);
+    expect(AlignmentGeometry.topCenter, Alignment.topCenter);
+    expect(AlignmentGeometry.topRight, Alignment.topRight);
+    expect(AlignmentGeometry.centerLeft, Alignment.centerLeft);
+    expect(AlignmentGeometry.center, Alignment.center);
+    expect(AlignmentGeometry.centerRight, Alignment.centerRight);
+    expect(AlignmentGeometry.bottomLeft, Alignment.bottomLeft);
+    expect(AlignmentGeometry.bottomCenter, Alignment.bottomCenter);
+    expect(AlignmentGeometry.bottomRight, Alignment.bottomRight);
+  });
 }