Support extends/implements of Function.

sdk/lib/_internal/compiler/implementation/resolution/members.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.
 
 part of resolution;
 
 abstract class TreeElements {
   Element get currentElement;
   Set<Node> get superUses;
 
@@ skipping 3931 matching lines @@
     return
       !identical(lib, compiler.coreLibrary) &&
       !identical(lib, compiler.jsHelperLibrary) &&
       !identical(lib, compiler.interceptorsLibrary) &&
       (identical(type, compiler.types.dynamicType) ||
        identical(type.element, compiler.boolClass) ||
        identical(type.element, compiler.numClass) ||
        identical(type.element, compiler.intClass) ||
        identical(type.element, compiler.doubleClass) ||
        identical(type.element, compiler.stringClass) ||
-       identical(type.element, compiler.nullClass) ||
-       identical(type.element, compiler.functionClass));
+       identical(type.element, compiler.nullClass));
   }
 }
 
 class ClassSupertypeResolver extends CommonResolverVisitor {
   Scope context;
   ClassElement classElement;
 
   ClassSupertypeResolver(Compiler compiler, ClassElement cls)
     : context = Scope.buildEnclosingScope(cls),
       this.classElement = cls,
@@ skipping 556 matching lines @@
     return finishConstructorReference(visit(expression),
                                       expression, expression);
   }
 }
 
 /// Looks up [name] in [scope] and unwraps the result.
 Element lookupInScope(Compiler compiler, Node node,
                       Scope scope, SourceString name) {
   return Elements.unwrap(scope.lookup(name), compiler, node);
 }