[es6] don't "replace" Object.prototype.toString for --harmony-tostring

test/mjsunit/function-arguments-null.js

Index: test/mjsunit/function-arguments-null.js
diff --git a/test/mjsunit/function-arguments-null.js b/test/mjsunit/function-arguments-null.js
index 21e542fc81e7a93362a886810ad7076443efecc5..6b47a10188b8ab2a97bca2d1731dec4241523eaa 100644
--- a/test/mjsunit/function-arguments-null.js
+++ b/test/mjsunit/function-arguments-null.js
@@ -25,6 +25,9 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// The arguments property of functions should be null when not
+// The arguments property of sloppy functions should be null when not
 // executing inside the function.
-assertTrue(toString.arguments === null);
+
+function sloppy() {}
+
+assertTrue(sloppy.arguments === null);