Moving StartWith to AsyncEnumerableEx.

Ix.NET/Source/System.Interactive.Async.Tests/AsyncTests.Single.cs

@@ -2946,8 +2946,8 @@ namespace Tests
         [Fact]
         public void StartWith_Null()
         {
-            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.StartWith(default(IAsyncEnumerable<int>), new[] { 1 }));
-            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.StartWith(AsyncEnumerable.Return(42), null));
+            AssertThrows<ArgumentNullException>(() => AsyncEnumerableEx.StartWith(default(IAsyncEnumerable<int>), new[] { 1 }));
+            AssertThrows<ArgumentNullException>(() => AsyncEnumerableEx.StartWith(AsyncEnumerable.Return(42), null));
         }
 
         [Fact]

Ix.NET/Source/System.Interactive.Async/StartWith.cs → Ix.NET/Source/System.Interactive.Async/System/Linq/Operators/StartWith.cs

@@ -6,7 +6,7 @@ using System.Collections.Generic;
 
 namespace System.Linq
 {
-    public static partial class AsyncEnumerable
+    public static partial class AsyncEnumerableEx
     {
         public static IAsyncEnumerable<TSource> StartWith<TSource>(this IAsyncEnumerable<TSource> source, params TSource[] values)
         {