Fixing some test code.

Ix.NET/Source/System.Linq.Async.Tests/System/Linq/Operators/DefaultIfEmpty.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information. 
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using Xunit;