Fix typing spotted by checked mode.

lib/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.
 
 class World {
   final Map<ClassElement, Set<ClassElement>> subtypes;
 
   World() : subtypes = new Map<ClassElement, Set<ClassElement>>();
 
   void populate(Compiler compiler, Collection<LibraryElement> libraries) {
     void addSubtypes(ClassElement cls) {
       for (Type type in cls.allSupertypes) {
-        List<Element> subtypes = subtypes.putIfAbsent(
+        Set<Element> subtypes = subtypes.putIfAbsent(
           type.element,
-          () => <ClassElement>[]);
+          () => new Set<ClassElement>());
         subtypes.add(cls);
       }
     }
 
     libraries.forEach((LibraryElement library) {
       for (Link<Element> link = library.topLevelElements;
            !link.isEmpty();
            link = link.tail) {
         Element element = link.head;
         if (!element.isClass()) continue;
@@ skipping 42 matching lines @@
   void add(Element element) {
     elements.add(element);
   }
 
   bool isEmpty() => elements.isEmpty();
 
   bool hasJustFields() {
     return elements.every((Element element) => element.isField());
   }
 }