[runtime] Make %FunctionGetScript and %FunctionGetSourceCode robust.

test/mjsunit/regress/regress-crbug-582703.js

Index: test/mjsunit/regress/regress-crbug-582703.js
diff --git a/test/message/arrow-invalid-rest.js b/test/mjsunit/regress/regress-crbug-582703.js
similarity index 66%
copy from test/message/arrow-invalid-rest.js
copy to test/mjsunit/regress/regress-crbug-582703.js
index 870dbe9f542ea95050514227feaf9b2299ca516d..792266f54cd9d124b2d2675100d3d1e8b82c4dd3 100644
--- a/test/message/arrow-invalid-rest.js
+++ b/test/mjsunit/regress/regress-crbug-582703.js
@@ -1,8 +1,8 @@
 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
-//
-//
 
-var f = (...x = 10) => x;
-f(1, 2, 3, 4, 5);
+// Flags: --allow-natives-syntax
+
+%FunctionGetScript({});
+%FunctionGetSourceCode({});