Adding class modifiers to dart:math.

Just adds `interface` modifier to `Random`.

Change-Id: I221a97a3c941befb9039a14d6ba827ab095837ac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/288207
Reviewed-by: Nate Bosch <nbosch@google.com>
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

CHANGELOG.md

@@ -139,6 +139,13 @@
   removed.  See [#49536](https://github.com/dart-lang/sdk/issues/49536) for
   details.
 
+#### `dart:math`
+
+- **Breaking change when migrating code to Dart 3.0**:
+  Some changes to platform libraries only affect code when it is migrated
+  to language version 3.0.
+  - The `Random` interface can only be implemented, not extended.
+
 #### `dart:io`
 
 - Deprecated `NetworkInterface.listSupported`. Has always returned true since

sdk/lib/math/random.dart

@@ -28,7 +28,7 @@ part of dart.math;
 /// var boolValue = Random().nextBool(); // true or false, with equal chance.
 /// ```
 ///
-abstract class Random {
+abstract interface class Random {
   /// Creates a random number generator.
   ///
   /// The optional parameter [seed] is used to initialize the