The Tor Browser / file revision

 gczeal(11);

 var otherGlobal = newGlobal();

 function test(str, arg, result)

 {

         var fun = new Function('x', str);

             var code = "(function (x) { " + str + " })";

                 var c = clone(otherGlobal.evaluate(code, {compileAndGo: false}));

                     assertEq(c.toSource(), eval(code).toSource());

 }

 test('return let (y) x;');

 test('return let (x) "" + x;', 'unicorns', 'undefined');

 test('return let (y = x) (y++, "" + y);', 'unicorns', 'NaN');

 test('return let (y = 1) (y = x, y);');

 test('return let ([] = x) x;');

 test('return let ([, ] = x) x;');

 test('return let ([, , , , ] = x) x;');

 test('return let ([[]] = x) x;');

 test('return let ([[[[[[[[[[[[[]]]]]]]]]]]]] = x) x;');

 test('return let ([[], []] = x) x;');

 test('return let ([[[[]]], [], , [], [[]]] = x) x;');