The Tor Browser / file revision

 // Test the JS shell's toy principals.

 var count = 0;

 // Given a string of letters |expected|, say "abc", assert that the stack

 // contains calls to a series of functions named by the next letter from

 // the string, say a, b, and then c. Younger frames appear earlier in

 // |expected| than older frames.

 function check(expected, stack) {

   print("check(" + uneval(expected) + ") against:\n" + stack);

   count++;

   // Extract only the function names from the stack trace. Omit the frames

   // for the top-level evaluation, if it is present.

   var split = stack.split(/(.)?@.*\n/).slice(0, -1);

   if (split[split.length - 1] === undefined)

     split = split.slice(0, -2);

   // Check the function names against the expected sequence.

   assertEq(split.length, expected.length * 2);

   for (var i = 0; i < expected.length; i++)

     assertEq(split[i * 2 + 1], expected[i]);

 }

 var low = newGlobal({ principal: 0 });

 var mid = newGlobal({ principal: 0xffff });

 var high = newGlobal({ principal: 0xfffff });

      eval('function a() { check("a",        Error().stack); b(); }');

 low .eval('function b() { check("b",        Error().stack); c(); }');

 mid .eval('function c() { check("cba",      Error().stack); d(); }');

 high.eval('function d() { check("dcba",     Error().stack); e(); }');

      eval('function e() { check("edcba",    Error().stack); f(); }'); // no principal, so checks skipped

 low .eval('function f() { check("fb",       Error().stack); g(); }');

 mid .eval('function g() { check("gfecba",   Error().stack); h(); }');

 high.eval('function h() { check("hgfedcba", Error().stack);      }');

 // Make everyone's functions visible to each other, as needed.

      b = low .b;

 low .c = mid .c;

 mid .d = high.d;

 high.e =      e;

      f = low .f;

 low .g = mid .g;

 mid .h = high.h;

 low.check = mid.check = high.check = check;

 // Kick the whole process off.

 a();

 assertEq(count, 8);