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)
@@ -91,10 +91,6 @@
     });
   }
 
-  bool needsRti(ClassElement cls) {
-    return classesNeedingRti.contains(cls) || compiler.enabledRuntimeType;
-  }
-
   void registerMixinUse(MixinApplicationElement mixinApplication,
                         ClassElement mixin) {
     Set<MixinApplicationElement> users =
@@ -103,6 +99,12 @@
     users.add(mixinApplication);
   }
 
+  bool isUsedAsMixin(ClassElement cls) {
+    Set<MixinApplicationElement> uses = mixinUses[cls];
+    return uses != null && !uses.isEmpty;
+  }
+
+
   void registerRtiDependency(Element element, Element dependency) {
     // We're not dealing with typedef for now.
     if (!element.isClass() || !dependency.isClass()) return;
@@ -111,6 +113,10 @@
     classes.add(dependency);
   }
 
+  bool needsRti(ClassElement cls) {
+    return classesNeedingRti.contains(cls) || compiler.enabledRuntimeType;
+  }
+
   void recordUserDefinedGetter(Element element) {
     assert(element.isGetter());
     userDefinedGetters.add(element);