Implement least upper bound.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 042e5c828608d2a7e55545fa0d626479c53560a0..452f366a473d13e247e15e44eb9232648682836f 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -29,6 +29,8 @@ import '../dart_types.dart';
 
 import '../scanner/scannerlib.dart' show Token, EOF_TOKEN;
 
+import '../ordered_typeset.dart' show OrderedTypeSet;
+
 
 class ElementX implements Element {
   static int elementHashCode = 0;
@@ -1694,7 +1696,11 @@ abstract class BaseClassElementX extends ElementX implements ClassElement {
   // compilation. They don't have any user-side counter-part.
   Link<Element> backendMembers = const Link<Element>();
 
-  Link<DartType> allSupertypes;
+  OrderedTypeSet allSupertypesAndSelf;
+
+  Link<DartType> get allSupertypes => allSupertypesAndSelf.supertypes;
+
+  int get hierarchyDepth => allSupertypesAndSelf.maxDepth;
 
   BaseClassElementX(String name,
                     Element enclosing,