Make sure the fast case of ScanJsonString bails out to the slow case correctly.

test/mjsunit/json.js

 // Copyright 2009 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
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 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
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@@ skipping 410 matching lines @@
 // objects in JSON.parse. Instead we read them as we would JS object
 // literals. If we change that, this test should change with it.
 //
 // Parse a non-object value as __proto__. This must not create a
 // __proto__ property different from the original, and should not
 // change the original.
 var o = JSON.parse('{"__proto__":5}');
 assertEquals(Object.prototype, o.__proto__);  // __proto__ isn't changed.
 assertEquals(0, Object.keys(o).length);  // __proto__ isn't added as enumerable.
 
-
-
+var json = '{"stuff before slash\\\\stuff after slash":"whatever"}';
+assertEquals(json, JSON.stringify(JSON.parse(json)));