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

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

Index: pkg/compiler/lib/src/elements/elements.dart
diff --git a/pkg/compiler/lib/src/elements/elements.dart b/pkg/compiler/lib/src/elements/elements.dart
index 442e12b22701773a76c374dde646b3d4dcc2f264..4c0c0396c7d378f5091822bf3ee525e6d1cdf471 100644
--- a/pkg/compiler/lib/src/elements/elements.dart
+++ b/pkg/compiler/lib/src/elements/elements.dart
@@ -1335,11 +1335,15 @@ abstract class ClassElement extends TypeDeclarationElement
   /// 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);