Add test for constant factory constructor feature, cf. #46984

Change-Id: I750345600d3b47de44c060511be35aea8744eca3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/211140
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

tests/language/const/factory_member_test.dart

@@ -0,0 +1,46 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:math';
+import 'package:expect/expect.dart';
+
+abstract class A {
+  A.named(this.x5);
+  const factory A() = B;
+
+  // No constant expression will evaluate to an instance of `A`, so
+  // there is no need to live up to the constant related constraints,
+  // we can have instance variables of various kinds, even mutable:
+
+  final List<Never> x1 = [];
+  abstract String x2;
+  late final int x3 = Random().nextInt(10000);
+  late final int x4;
+  double? x5;
+}
+
+class B implements A {
+  const B([this.x5 = 0.57721566490153286]) : x1 = const [];
+
+  // Implement the interface of `A` appropriately for a constant.
+
+  final List<Never> x1;
+
+  final String x2 = 'B.x2';
+  set x2(String _) {}
+
+  int get x3 => 42;
+
+  int get x4 => -42;
+  set x4(_) {}
+
+  final double? x5;
+  set x5(double? _) {}
+}
+
+void main() {
+  const A();
+  const B(2.1);
+  Expect.isTrue(identical(const B(0.57721566490153286), const A()));
+}