Implement least upper bound.

sdk/lib/_internal/compiler/implementation/closure.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.
 
 library closureToClassMapper;
 
 import "elements/elements.dart";
 import "dart2jslib.dart";
 import "dart_types.dart";
 import "scanner/scannerlib.dart" show Token;
@@ skipping 113 matching lines @@
               // is unique, but also emit closure classes after all other
               // classes (since the emitter sorts classes by their id).
               compiler.getNextFreeClassId(),
               STATE_DONE) {
     ClassElement superclass = methodElement.isInstanceMember()
         ? compiler.boundClosureClass
         : compiler.closureClass;
     superclass.ensureResolved(compiler);
     supertype = superclass.computeType(compiler);
     interfaces = const Link<DartType>();
-    allSupertypes = const Link<DartType>().prepend(supertype);
     thisType = rawType = new InterfaceType(this);
+    allSupertypesAndSelf =
+        superclass.allSupertypesAndSelf.extendClass(thisType);
   }
 
   bool isClosure() => true;
 
   Token position() => node.getBeginToken();
 
   Node parseNode(DiagnosticListener listener) => node;
 
   /**
    * The most outer method this closure is declared into.
@@ skipping 684 matching lines @@
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     bool oldInTryStatement = inTryStatement;
     inTryStatement = true;
     node.visitChildren(this);
     inTryStatement = oldInTryStatement;
   }
 }