[TypeSystem] UsingScope now uses as operator for casts.

ICSharpCode.NRefactory.CSharp/TypeSystem/UsingScope.cs

@@ -163,7 +163,7 @@ namespace ICSharpCode.NRefactory.CSharp.TypeSystem
 		public ResolvedUsingScope Resolve(ICompilation compilation)
 		{
 			CacheManager cache = compilation.CacheManager;
-			ResolvedUsingScope resolved = (ResolvedUsingScope)cache.GetShared(this);
+			ResolvedUsingScope resolved = cache.GetShared(this) as ResolvedUsingScope;
 			if (resolved == null) {
 				var csContext = new CSharpTypeResolveContext(compilation.MainAssembly, parent != null ? parent.Resolve(compilation) : null);
 				resolved = (ResolvedUsingScope)cache.GetOrAddShared(this, new ResolvedUsingScope(csContext, this));