Fix several bugs in error stack trace formatting.

test/mjsunit/stack-traces.js

 // Copyright 2009 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 292 matching lines @@
 Array.prototype.push = function(x) { assertUnreachable(); };
 Array.prototype.join = function(x) { assertUnreachable(); };
 error.stack;
 
 var fired = false;
 error = new Error({ toString: function() { fired = true; } });
 assertTrue(fired);
 error.stack;
 assertTrue(fired);
 
-//Check that throwing exception in a custom stack trace formatting function
-//does not lead to recursion.
-Error.prepareStackTrace = function() { throw new Error("abc"); }
+// Check that throwing exception in a custom stack trace formatting function
+// does not lead to recursion.
+Error.prepareStackTrace = function() { throw new Error("abc"); };
 var message;
 try {
   throw new Error();
 } catch (e) {
   message = e.message;
 }
 
 assertEquals("abc", message);
+
+// Test that modifying Error.prepareStackTrace by itself works.
+Error.prepareStackTrace = function() { Error.prepareStackTrace = "custom"; };
+new Error();
+
+assertEquals("custom", Error.prepareStackTrace);