Moving OfType to its own file.

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Cast.cs

@@ -21,13 +21,5 @@ namespace System.Linq
 
             return source.Select(x => (TResult)x);
         }
-
-        public static IAsyncEnumerable<TType> OfType<TType>(this IAsyncEnumerable<object> source)
-        {
-            if (source == null)
-                throw new ArgumentNullException(nameof(source));
-
-            return source.Where(x => x is TType).Cast<TType>();
-        }
     }
 }

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/OfType.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+using System.Collections.Generic;
+
+namespace System.Linq
+{
+    public static partial class AsyncEnumerable
+    {
+        public static IAsyncEnumerable<TType> OfType<TType>(this IAsyncEnumerable<object> source)
+        {
+            if (source == null)
+                throw new ArgumentNullException(nameof(source));
+
+            return source.Where(x => x is TType).Cast<TType>();
+        }
+    }
+}