The Tor Browser / file revision

 // If the offset parameter to setBreakpoint is invalid, throw an error.

 load(libdir + "asserts.js");

 var g = newGlobal();

 var dbg = new Debugger(g);

 var hits = 0;

 dbg.onDebuggerStatement = function (frame) {

     // We assume at least one offset between 0 and frame.offset is invalid.

     assertThrowsInstanceOf(

         function () {

             for (var i = 0; i < frame.offset; i++)

                 frame.script.setBreakpoint(i, {});

         },

         Error);

     hits++;

 };

 g.eval("x = 256; debugger;");

 assertEq(hits, 1);