Use 'using SD = System.Diagnostics' to avoid ambiguous references.

mcs/class/corlib/Test/System.Threading/ThreadTest.cs

@@ -15,7 +15,7 @@ using System.Security.Principal;
 using System.Threading;
 using System.Reflection;
 using System.Collections.Generic;
-using System.Diagnostics;
+using SD = System.Diagnostics;
 
 using NUnit.Framework;
 
@@ -1257,7 +1257,7 @@ namespace MonoTests.System.Threading
 		public void GetStackTraces () {
 			var m = typeof (Thread).GetMethod ("Mono_GetStackTraces", BindingFlags.NonPublic|BindingFlags.Static);
 			if (m != null) {
-				var res = (Dictionary<Thread,StackTrace>)typeof (Thread).GetMethod ("Mono_GetStackTraces", BindingFlags.NonPublic|BindingFlags.Static).Invoke (null, null);
+				var res = (Dictionary<Thread,SD.StackTrace>)typeof (Thread).GetMethod ("Mono_GetStackTraces", BindingFlags.NonPublic|BindingFlags.Static).Invoke (null, null);
 				foreach (var t in res.Keys) {
 					var st = res [t].ToString ();
 				}