[io] Make FileSystemEvent sealed

Bug: https://github.com/dart-lang/sdk/issues/52027, https://github.com/dart-lang/sdk/issues/51912
Change-Id: I154ffe5901e4248f48400e6c84568c9fe6dbcd83
CoreLibraryReviewExempt: aske on holiday
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/302452
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Siva Annamalai <asiva@google.com>

CHANGELOG.md

@@ -10,17 +10,22 @@
    current directory ([#39796][]).
 
 [#39796]: https://darbug.com/39796
+
 #### `dart:io`
 
 - **Breaking change** [#51486][]:
   - Added `sameSite` to the `Cookie` class.
   - Added class `SameSite`.
+- **Breaking change** [#52027][]: `FileSystemEvent` is
+  [`sealed`](https://dart.dev/language/class-modifiers#sealed). This means
+  that `FileSystemEvent` cannot be extended or implemented.
 - Added a deprecation warning when `Platform` is instantiated.
 - Added `Platform.lineTerminator` which exposes the character or characters
   that the operating system uses to separate lines of text, e.g.,
   `"\r\n"` on Windows.
 
 [#51486]: https://github.com/dart-lang/sdk/issues/51486
+[#52027]: https://github.com/dart-lang/sdk/issues/52027
 
 ### Other libraries
 

sdk/lib/io/file_system_entity.dart

@@ -876,7 +876,7 @@ abstract class FileSystemEntity {
 }
 
 /// Base event class emitted by [FileSystemEntity.watch].
-final class FileSystemEvent {
+sealed class FileSystemEvent {
   /// Bitfield for [FileSystemEntity.watch], to enable [FileSystemCreateEvent]s.
   static const int create = 1 << 0;