Avoid emmitting is test for Function if superclass already has it.

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import 'elements.dart';
 import '../constants/expressions.dart';
 import '../helpers/helpers.dart';  // Included for debug helpers.
 import '../tree/tree.dart';
@@ skipping 2267 matching lines @@
       }
     }
 
     forEachMember(fieldFilter);
   }
 
   void forEachBackendMember(void f(Element member)) {
     backendMembers.forEach(f);
   }
 
+  bool implementsFunction(Compiler compiler) {
+    return asInstanceOf(compiler.functionClass) != null || callType != null;
+  }
+
   bool implementsInterface(ClassElement intrface) {
     for (DartType implementedInterfaceType in allSupertypes) {
       ClassElement implementedInterface = implementedInterfaceType.element;
       if (identical(implementedInterface, intrface)) {
         return true;
       }
     }
     return false;
   }
 
@@ skipping 414 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }