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)
@@ -148,20 +148,10 @@
   }
 
   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)
-        ? compiler.objectClass
-        : mask.base.element;
-    // 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;
+    ti.TypeMask mask = selector.mask == null
+        ? new ti.TypeMask.subclass(compiler.objectClass.rawType)
+        : selector.mask;
+    return mask.locateSingleElement(selector, compiler);
   }
 
   bool hasSingleMatch(Selector selector) {