The Tor Browser / file revision

 // A breakpoint handler hit method can return {throw: exc} to throw an exception.

 var g = newGlobal();

 var dbg = Debugger(g);

 dbg.onDebuggerStatement = function (frame) {

     function hit(frame) {

         return {throw: frame.eval("new Error('PASS')").return};

     }

     var s = frame.script;

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

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

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

 };

 g.s = null;

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

        "debugger;\n" +          // line0 + 1

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

        "    s = 'FAIL';\n" +    // line0 + 3

        "} catch (exc) {\n" +

        "    assertEq(exc.constructor, Error);\n" +

        "    s = exc.message;\n" +

        "}\n");

 assertEq(g.s, 'PASS');