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

pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
index 5fc4d0cae242c9f5f54a8cdca0456b9e3bd5334d..aa88a7a043599636f34fe8835510a72a04921424 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/type_test_emitter.dart
@@ -168,10 +168,14 @@ class TypeTestEmitter extends CodeEmitterHelper {
         call = cls.lookupBackendMember(Compiler.CALL_OPERATOR_NAME);
       }
       if (call != null && call.isFunction) {
-        generateInterfacesIsTests(compiler.functionClass,
-                                  emitIsTest,
-                                  emitSubstitution,
-                                  generated);
+        // A superclass might already implement the Function interface. In such
+        // a case, we can avoid emiting the is test here.
+        if (!cls.superclass.implementsFunction(compiler)) {
+          generateInterfacesIsTests(compiler.functionClass,
+                                    emitIsTest,
+                                    emitSubstitution,
+                                    generated);
+        }
         FunctionType callType = call.computeType(compiler);
         Map<FunctionType, bool> functionTypeChecks =
             getFunctionTypeChecksOn(callType);