Make formatting error message side-effect-free.

test/mjsunit/regress/regress-2398.js

Index: test/mjsunit/regress/regress-2398.js
diff --git a/test/mjsunit/regress/regress-136048.js b/test/mjsunit/regress/regress-2398.js
similarity index 84%
copy from test/mjsunit/regress/regress-136048.js
copy to test/mjsunit/regress/regress-2398.js
index c9972e96fc062b4618f8bf40ee83c15f185ff17c..1c66e7f84c041695c2718ce655fbe47ae38b1866 100644
--- a/test/mjsunit/regress/regress-136048.js
+++ b/test/mjsunit/regress/regress-2398.js
@@ -25,10 +25,17 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+"use strict";
+
+var observed = false;
+
+var object = { get toString() { observed = true; } };
+Object.defineProperty(object, "ro", { value: 1 });
+
 try {
-  /foo/\u0069
+  object.ro = 2;  // TypeError caused by trying to write to read-only.
 } catch (e) {
-  assertEquals(
-      "SyntaxError: Invalid flags supplied to RegExp constructor '\\u0069'",
-      e.toString());
+  e.message;  // Forces formatting of the message object.
 }
+
+assertFalse(observed);