Allow private selectors to be typed, and fix ClassElement.isSubclassOf to take patches into account.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/elements/modelx.dart	(revision 18125)
+++ sdk/lib/_internal/compiler/implementation/elements/modelx.dart	(working copy)
@@ -1700,7 +1700,10 @@
    * account.
    */
   bool isSubclassOf(ClassElement cls) {
-    for (ClassElement s = this; s != null; s = s.superclass) {
+    // Use [declaration] for both [this] and [cls], because
+    // declaration classes hold the superclass hierarchy.
+    cls = cls.declaration;
+    for (ClassElement s = declaration; s != null; s = s.superclass) {
       if (identical(s, cls)) return true;
     }
     return false;