Add a type kind to TypedSelector. A typed selector can either be

sdk/lib/_internal/compiler/implementation/world.dart

Index: sdk/lib/_internal/compiler/implementation/world.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/world.dart	(revision 18306)
+++ sdk/lib/_internal/compiler/implementation/world.dart	(working copy)
@@ -25,6 +25,11 @@
         rtiDependencies = new Map<ClassElement, Set<ClassElement>>(),
         this.compiler = compiler;
 
+  bool isUsedAsMixin(ClassElement cls) {

[kasperl, 2013/02/11 14:30:34]

Move closer to registerMixinUse.

[ngeoffray, 2013/02/11 14:53:29]

Done.

+    Set<MixinApplicationElement> uses = mixinUses[cls];
+    return uses != null && !uses.isEmpty;
+  }
+
   void populate() {
     void addSubtypes(ClassElement cls) {
       if (cls.resolutionState != STATE_DONE) {