Fix one crash in the compiler.

frog/leg/typechecker.dart

Index: frog/leg/typechecker.dart
===================================================================
--- frog/leg/typechecker.dart	(revision 4934)
+++ frog/leg/typechecker.dart	(working copy)
@@ -406,6 +406,9 @@
         if (receiverType === null) {
           fail(node.receiver, 'receivertype is null');
         }
+        if (receiverType.element.kind !== ElementKind.CLASS) {
+          fail(node.receiver, 'receivertype is not a class');
+        }
         ClassElement classElement = receiverType.element;
         // TODO(karlklose): substitute type arguments.
         Type memberType = lookupMethodType(node, classElement, selector.source);