Imported Upstream version 6.4.0.137

Former-commit-id: 943baa9f16a098c33e129777827f3a9d20da00d6

mcs/class/corlib/corefx/Stream.cs

@@ -0,0 +1,20 @@
+using System.Threading.Tasks;
+
+namespace System.IO
+{
+    partial class Stream : IAsyncDisposable
+    {
+        public virtual ValueTask DisposeAsync()
+        {
+            try
+            {
+                Dispose();
+                return default;
+            }
+            catch (Exception exc)
+            {
+                return new ValueTask(Task.FromException(exc));
+            }
+        }
+    }
+}