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

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

Index: sdk/lib/_internal/compiler/implementation/world.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/world.dart	(revision 22167)
+++ sdk/lib/_internal/compiler/implementation/world.dart	(working copy)
@@ -152,11 +152,10 @@
     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