Avoid repeated delegate allocations in FileStream.FlushAsync (dotnet/coreclr#8048)

Avoid the `Action<object>` allocation on each invocation of
`FileStream.FlushAsync` on Windows. This change makes the Win32
implementation match Unix.

Commit migrated from https://github.com/dotnet/coreclr/commit/2d19d2e7d57deee034650a8cc078c4ad93ce03c8

diff --git a/src/coreclr/src/mscorlib/corefx/System/IO/FileStream.Win32.cs b/src/coreclr/src/mscorlib/corefx/System/IO/FileStream.Win32.cs
index 8b9adb5..350d948 100644 (file)
--- a/src/coreclr/src/mscorlib/corefx/System/IO/FileStream.Win32.cs
+++ b/src/coreclr/src/mscorlib/corefx/System/IO/FileStream.Win32.cs
@@ -1715,7 +1715,7 @@ namespace System.IO
             if (CanWrite)
             {
                 return Task.Factory.StartNew(
-                    state => FlushOSBuffer(),
+                    state => ((FileStream)state).FlushOSBuffer(),
                     this,
                     cancellationToken,
                     TaskCreationOptions.DenyChildAttach,