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;
   final Map<ClassElement, Set<ClassElement>> typesImplementedBySubclasses;
   final Set<ClassElement> classesNeedingRti;
   final Map<ClassElement, Set<ClassElement>> rtiDependencies;
   final FunctionSet userDefinedGetters;
   final FunctionSet userDefinedSetters;
 
   World(Compiler compiler)
       : subtypes = new Map<ClassElement, Set<ClassElement>>(),
         mixinUses = new Map<ClassElement, Set<MixinApplicationElement>>(),
         typesImplementedBySubclasses =
             new Map<ClassElement, Set<ClassElement>>(),
         userDefinedGetters = new FunctionSet(compiler),
         userDefinedSetters = new FunctionSet(compiler),
         classesNeedingRti = new Set<ClassElement>(),
         rtiDependencies = new Map<ClassElement, Set<ClassElement>>(),
         this.compiler = compiler;
 
+  bool isUsedAsMixin(ClassElement cls) {
+    Set<MixinApplicationElement> uses = mixinUses[cls];
+    return uses != null && !uses.isEmpty;
+  }
+
   void populate() {
     void addSubtypes(ClassElement cls) {
       if (cls.resolutionState != STATE_DONE) {
         compiler.internalErrorOnElement(
             cls, 'Class "${cls.name.slowToString()}" is not resolved.');
       }
 
       for (DartType type in cls.allSupertypes) {
         Set<Element> subtypesOfCls =
           subtypes.putIfAbsent(type.element, () => new Set<ClassElement>());
@@ skipping 185 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;
 }