[ dart:io ] Breaking change: mark IOOverrides as abstract base

Fixes #56468

CoreLibraryReviewExempt: Only impacts dart:io
Change-Id: I529ac7bbb50640cb1a0fba967cf0bcc6e3bb52aa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/389520
Reviewed-by: Siva Annamalai <[email protected]>
Reviewed-by: Jessy Yameogo <[email protected]>
Commit-Queue: Ben Konyi <[email protected]>
Auto-Submit: Ben Konyi <[email protected]>

Author: bkonyi

CHANGELOG.md

@@ -20,6 +20,14 @@
   Dart SDK for ia32 anymore (support to execute the `dartvm` for ia32
   architecture is retained)
 
+### Libraries
+
+#### `dart:io`
+
+- **Breaking Change** [#56468][]: Marked `IOOverrides` as an `abstract base`
+  class.
+
+[#56468]: https://github.com/dart-lang/sdk/issues/56468
 
 ## 3.9.0
 
@@ -890,6 +898,7 @@ The AOT snapshot can be used as follows to run DDC:
 [pub#4445]: https://github.com/dart-lang/pub/issues/4445
 [#57084]: https://github.com/dart-lang/sdk/issues/57084
 [#56552]: https://github.com/dart-lang/sdk/issues/56552
+>>>>>>> BASE      (d73f7bdb4366b447eb9874fe80381b8efd03c718 Generate `GeneralizingAstVisitor`, `TimedAstVisitor`, `Analy)
 
 ## 3.6.0
 

sdk/lib/io/overrides.dart

@@ -29,7 +29,7 @@ final _ioOverridesToken = Object();
 ///   }, createDirectory: (String path) => new MyDirectory(path));
 /// }
 /// ```
-abstract class IOOverrides {
+abstract base class IOOverrides {
   static IOOverrides? _global;
 
   static IOOverrides? get current {
@@ -379,7 +379,7 @@ abstract class IOOverrides {
   }
 }
 
-class _IOOverridesScope extends IOOverrides {
+final class _IOOverridesScope extends IOOverrides {
   final IOOverrides? _previous;
 
   // Directory

tests/standalone/io/io_override_test.dart

@@ -305,7 +305,7 @@ Future<Null> ioOverridesRunTest() async {
   await f;
 }
 
-class MyIOOverrides extends IOOverrides {
+final class MyIOOverrides extends IOOverrides {
   Directory createDirectory(String path) => DirectoryMock.createDirectory(path);
 }
 
@@ -331,7 +331,7 @@ globalIOOverridesZoneTest() {
   Expect.isTrue(dir is Directory);
 }
 
-class EmptyOverride extends IOOverrides {}
+final class EmptyOverride extends IOOverrides {}
 
 void emptyIOOverride() {
   IOOverrides.runWithIOOverrides(