Fix issue 6472 that exhibited a few crashing bugs in dart2js.

lib/compiler/implementation/universe/partial_type_tree.dart

Index: lib/compiler/implementation/universe/partial_type_tree.dart
===================================================================
--- lib/compiler/implementation/universe/partial_type_tree.dart	(revision 14453)
+++ lib/compiler/implementation/universe/partial_type_tree.dart	(working copy)
@@ -63,7 +63,9 @@
   // TODO(kasperl): Move this to the Selector class?
   ClassElement selectorType(Selector selector) {
     DartType type = selector.receiverType;
-    return (type != null) ? type.element : compiler.objectClass;
+    if (type == null) return compiler.objectClass;
+    if (type.element.isTypedef()) return compiler.functionClass;
+    return type.element;
   }
 
   /**