The Tor Browser / file revision

 // A breakpoint handler hit method can return null to raise an uncatchable error.

 var g = newGlobal();

 var dbg = Debugger(g);

 dbg.onDebuggerStatement = function (frame) {

     g.log += 'D';

     function hit(frame) {

         g.log += 'H';

         return null;

     }

     var f = frame.eval("f").return;

     var s = f.script;

     var offs = s.getLineOffsets(g.line0 + 3);

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

         s.setBreakpoint(offs[i], {hit: hit});

     var rv = f.call();

     assertEq(rv, null);

     g.log += 'X';

 };

 g.log = '';

 g.eval("line0 = Error().lineNumber;\n" +

        "function f() {\n" +         // line0 + 1

        "    try {\n" +              // line0 + 2

        "        log += '3';\n" +    // line0 + 3

        "    } catch (exc) {\n" +

        "        log += '5';\n" +

        "    }\n" +

        "}\n" +

        "debugger;\n");

 assertEq(g.log, 'DHX');