[io] Calling flush after close on File.open's IOSync should not throw an exception

The documentation of IOSync states that calling flush on a closed IOSink may have no effect:
https://github.com/dart-lang/sdk/blob/main/sdk/lib/io/io_sink.dart#L109
The future returned by close in the new implementation does not complete until after
a call to flush has completed.
The previous implementation of File.open did not throw when calling flush after close.

CoreLibraryReviewExempt: IO only, restoring previous semantics
Bug: b/274405250
Change-Id: I56bbe02e886085cc8156e60264f5b77593c8a075
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289823
Reviewed-by: Emmanuel Pellereau <emmanuelp@google.com>

sdk/lib/io/file_impl.dart

@@ -439,7 +439,7 @@ class _RandomAccessFileIOSync implements IOSink {
 
   Future<File?> flush() {
     if (_isClosed) {
-      throw StateError("${_file?.path ?? "IOSink"} is closed");
+      return done;
     }
     if (_isBound) {
       throw StateError("${_file?.path ?? "IOSink"} is bound to a stream");

tests/standalone/io/file_iosink_test.dart

@@ -392,13 +392,8 @@ Future<void> testFlushDuringClose(Directory tmpDir) async {
   final sink = file.openWrite();
   final close = sink.close();
 
-  try {
-    await sink.flush();
-  } on StateError catch (e) {
-    Expect.contains('flush_during_close is closed', e.message);
-  }
-
-  await close;
+  Expect.equals(file, await sink.flush());
+  Expect.equals(file, await close);
 }
 
 Future<void> testFailedWrite(Directory tmpDir) async {

tests/standalone_2/io/file_iosink_test.dart

@@ -394,13 +394,8 @@ Future<void> testFlushDuringClose(Directory tmpDir) async {
   final sink = file.openWrite();
   final close = sink.close();
 
-  try {
-    await sink.flush();
-  } on StateError catch (e) {
-    Expect.contains('flush_during_close is closed', e.message);
-  }
-
-  await close;
+  Expect.equals(file, await sink.flush());
+  Expect.equals(file, await close);
 }
 
 Future<void> testFailedWrite(Directory tmpDir) async {