Do not eagerly convert exception to string when creating a message object

test/mjsunit/regress/regress-crbug-490680.js

 // Copyright 2015 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 sentinel = null;
+
 try {
-  load("test/mjsunit/regress/regress-444805.js-script");
+  throw { toString: function() { sentinel = "observed"; } };
 } catch (e) {
 }

[Michael Starzinger, 2015/05/22 16:18:39]

Can we also add the following construct to the test case?

L: try {
  throw { toString: function() { sentinel = "observed"; } };
} finally {
  break L;
}

+
+assertNull(sentinel);