Normalize type masks to use the least upper instantiated subclass/type.

tests/try/poi/forget_element_test.dart

 // Copyright (c) 2014, 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.
 
 // Test of Compiler.forgetElement.
 library trydart.forget_element_test;
 
 import 'package:compiler/src/elements/elements.dart' show
     AstElement,
     ClassElement,
@@ skipping 242 matching lines @@
 
   Iterable resolutionMembersIn(LibraryElement library) {
     sameLibrary(e) => e.library == library;
     return new Set()
         ..addAll(resolutionUniverse.closurizedMembers.where(sameLibrary))
         ..addAll(resolutionUniverse.fieldSetters.where(sameLibrary))
         ..addAll(resolutionUniverse.fieldGetters.where(sameLibrary));
   }
 
   Iterable codegenSeenClassesIn(LibraryElement library) {
-    return compiler.codegenWorld.allInstantiatedClasses.where(
+    return compiler.enqueuer.codegen.processedClasses.where(
         (e) => e.library == library);
   }
 
   Iterable resolutionSeenClassesIn(LibraryElement library) {
-    return compiler.resolverWorld.allInstantiatedClasses.where(
+    return compiler.enqueuer.resolution.processedClasses.where(
         (e) => e.library == library);
   }
 }
 
 class NodeCollector extends tree.Visitor {
   final List<tree.Node> nodes = <tree.Node>[];
 
   void visitNode(tree.Node node) {
     nodes.add(node);
     node.visitChildren(this);
@@ skipping 116 matching lines @@
     // correctly (deeply nested function).
     new ForgetElementTestCase(
         'main() => (() => (([x = const Constant()]) => x)())();'
         ' $CONSTANT_CLASS',
         closureCount: 2,
         constantCount: 1,
         initialValueCount: 1),
 
     // TODO(ahe): Add test for super sends [backend.aliasedSuperMembers].
 ]..addAll(assertUnimplementedLocalMetadata());