almost done with the cleanup i think

Ix.NET/Source/System.Interactive.Async/ToAsyncEnumerable.cs

@@ -2,11 +2,9 @@
 // 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;
 using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -83,11 +81,13 @@ namespace System.Linq
         internal sealed class AsyncEnumerableAdapter<T> : AsyncIterator<T>, IIListProvider<T>
         {
             private readonly IEnumerable<T> source;
+
             private IEnumerator<T> enumerator;
  
             public AsyncEnumerableAdapter(IEnumerable<T> source)
             {
                 Debug.Assert(source != null);
+
                 this.source = source;
             }
 

Ix.NET/Source/System.Interactive.Async/Zip.cs

@@ -2,9 +2,8 @@
 // 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;
 using System.Collections.Generic;
-using System.Linq;
+using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -35,6 +34,10 @@ namespace System.Linq
 
             public ZipAsyncIterator(IAsyncEnumerable<TFirst> first, IAsyncEnumerable<TSecond> second, Func<TFirst, TSecond, TResult> selector)
             {
+                Debug.Assert(first != null);
+                Debug.Assert(second != null);
+                Debug.Assert(selector != null);
+
                 this.first = first;
                 this.second = second;
                 this.selector = selector;