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

pkg/compiler/lib/src/elements/elements.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 elements;
 
 
 import '../constants/expressions.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
@@ skipping 1317 matching lines @@
   String get nativeTagInfo;
 
   bool get isMixinApplication;
   bool get isUnnamedMixinApplication;
   bool get hasBackendMembers;
   bool get hasLocalScopeMembers;
 
   /// Returns `true` if this class is `Object` from dart:core.
   bool get isObject;
 
+  /// Returns `true` if this class implements [Function] either by directly
+  /// implementing the interface or by providing a [call] method.
+  bool implementsFunction(Compiler compiler);
+
   bool isSubclassOf(ClassElement cls);
   /// Returns true if `this` explicitly/nominally implements [intrface].
   ///
   /// Note that, if [intrface] is the `Function` class, this method returns
-  /// falso for a class that has a `call` method but does not explicitly
+  /// false for a class that has a `call` method but does not explicitly
   /// implement `Function`.
   bool implementsInterface(ClassElement intrface);
   bool hasFieldShadowedBy(Element fieldMember);
 
   /// Returns `true` if this class has a @proxy annotation.
   bool get isProxy;
 
   /// Returns `true` if the class hierarchy for this class contains errors.
   bool get hasIncompleteHierarchy;
 
@@ skipping 249 matching lines @@
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }