Make TypeMask an interface and start hiding implementation details of FlatTypeMask.

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<MixinApplicationElement>> mixinUses;
   final Map<ClassElement, Set<ClassElement>> typesImplementedBySubclasses;
@@ skipping 134 matching lines @@
   VariableElement locateSingleField(Selector selector) {
     Element result = locateSingleElement(selector);
     return (result != null && result.isField()) ? result : null;
   }
 
   Element locateSingleElement(Selector selector) {
     Iterable<Element> targets = allFunctions.filter(selector);
     if (targets.length != 1) return null;
     Element result = targets.first;
     ClassElement enclosing = result.getEnclosingClass();
-    // TODO(kasperl): Move this code to the type mask.
     ti.TypeMask mask = selector.mask;
-    ClassElement receiverTypeElement = (mask == null || mask.base == null)
+    ClassElement receiverTypeElement = (mask == null || mask.isEmpty)
         ? compiler.objectClass
-        : mask.base.element;
+        : mask.topClass();
     // We only return the found element if it is guaranteed to be
     // implemented on the exact receiver type. It could be found in a
     // subclass or in an inheritance-wise unrelated class in case of
     // subtype selectors.
     return (receiverTypeElement.isSubclassOf(enclosing)) ? result : null;
   }
 
   bool hasSingleMatch(Selector selector) {
     Iterable<Element> targets = allFunctions.filter(selector);
     return targets.length == 1;
@@ skipping 13 matching lines @@
   }
 
   void addFunctionCalledInLoop(Element element) {
     functionsCalledInLoop.add(element);
   }
 
   bool isCalledInLoop(Element element) {
     return functionsCalledInLoop.contains(element);
   }
 }