add missing GC.SuppressFinalize(this) to FileStream.DisposeAsync (#65899)

* add missing GC.SuppressFinalize(this) to FileStream.DisposeAsync

* DerivedFileStreamStrategy.DisposeAsync does not need to call _fileStream.BaseDisposeAsync, as FileStream.DisposeAsync calls Dispose(false) now

Author: adamsitnik

src/libraries/System.IO.FileSystem/tests/FileStream/DisposeAsync.cs

@@ -20,7 +20,6 @@ public async Task DisposeAsyncClosesHandle()
         }
 
         [Fact]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/65835")]
         public async Task DisposeAsyncFlushes()
         {
             string path = GetTestFilePath();

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs

@@ -497,7 +497,12 @@ public override long Position
         // _strategy can be null only when ctor has thrown
         protected override void Dispose(bool disposing) => _strategy?.DisposeInternal(disposing);
 
-        public override ValueTask DisposeAsync() => _strategy.DisposeAsync();
+        public async override ValueTask DisposeAsync()
+        {
+            await _strategy.DisposeAsync().ConfigureAwait(false);
+            Dispose(false);
+            GC.SuppressFinalize(this);
+        }
 
         public override void CopyTo(Stream destination, int bufferSize)
         {
@@ -576,8 +581,6 @@ internal Task BaseWriteAsync(byte[] buffer, int offset, int count, CancellationT
         internal ValueTask BaseWriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
             => base.WriteAsync(buffer, cancellationToken);
 
-        internal ValueTask BaseDisposeAsync() => base.DisposeAsync();
-
         internal Task BaseCopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
             => base.CopyToAsync(destination, bufferSize, cancellationToken);
 

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/DerivedFileStreamStrategy.cs

@@ -146,7 +146,7 @@ public override Task FlushAsync(CancellationToken cancellationToken)
         public override Task CopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
             => _fileStream.BaseCopyToAsync(destination, bufferSize, cancellationToken);
 
-        public override ValueTask DisposeAsync() => _fileStream.BaseDisposeAsync();
+        public override ValueTask DisposeAsync() => _strategy.DisposeAsync();
 
         protected sealed override void Dispose(bool disposing) => _strategy.DisposeInternal(disposing);
     }