Make sure error message formatting does not have side effects.

test/mjsunit/error-tostring.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 65 matching lines @@
 }
 
 assertEquals(["Error",[1,3]], testErrorToString(undefined, undefined));
 assertEquals(["e1",[1,2,3]], testErrorToString("e1", undefined));
 assertEquals(["e1: null",[1,2,3,4]], testErrorToString("e1", null));
 assertEquals(["e1",[1,2,3,4]], testErrorToString("e1", ""));
 assertEquals(["Error: e2",[1,3,4]], testErrorToString(undefined, "e2"));
 assertEquals(["null: e2",[1,2,3,4]], testErrorToString(null, "e2"));
 assertEquals(["e2",[1,2,3,4]], testErrorToString("", "e2"));
 assertEquals(["e1: e2",[1,2,3,4]], testErrorToString("e1", "e2"));
+
+var obj = {
+  get constructor () {
+    assertUnreachable();
+  }
+};
+
+assertThrows(function() { obj.x(); });