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

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js;
 
 class World {
   final Compiler compiler;
   final Map<ClassElement, Set<ClassElement>> subtypes;
   final Map<ClassElement, Set<MixinApplicationElement>> mixinUses;
@@ skipping 73 matching lines @@
     compiler.resolverWorld.isChecks.forEach((DartType type) {
       if (type is InterfaceType) {
         InterfaceType itf = type;
         if (!itf.isRaw) {
           potentiallyAddForRti(itf.element);
         }
       }
     });
   }
 
-  bool needsRti(ClassElement cls) {
-    return classesNeedingRti.contains(cls) || compiler.enabledRuntimeType;
-  }
-
   void registerMixinUse(MixinApplicationElement mixinApplication,
                         ClassElement mixin) {
     Set<MixinApplicationElement> users =
         mixinUses.putIfAbsent(mixin, () =>
                               new Set<MixinApplicationElement>());
     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;
     Set<ClassElement> classes =
         rtiDependencies.putIfAbsent(element, () => new Set<ClassElement>());
     classes.add(dependency);
   }
 
+  bool needsRti(ClassElement cls) {
+    return classesNeedingRti.contains(cls) || compiler.enabledRuntimeType;
+  }
+
   void recordUserDefinedGetter(Element element) {
     assert(element.isGetter());
     userDefinedGetters.add(element);
   }
 
   void recordUserDefinedSetter(Element element) {
     assert(element.isSetter());
     userDefinedSetters.add(element);
   }
 
@@ skipping 99 matching lines @@
   final SourceString name;
 
   MemberSet(SourceString this.name) : elements = new Set<Element>();
 
   void add(Element element) {
     elements.add(element);
   }
 
   bool get isEmpty => elements.isEmpty;
 }