Sort usings.

src/Compilers/CSharp/Portable/Binder/Binder.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
@@ -7,7 +8,6 @@
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Roslyn.Utilities;
-using System;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
@@ -807,4 +807,4 @@ TreeDumperNode DumpAncestors()
             }
         }
     }
-}
+}
\ No newline at end of file