The Tor Browser / file revision

 // Multiple Debugger objects can set breakpoints at the same instruction.

 var g = newGlobal();

 function attach(g, i) {

     var dbg = Debugger(g);

     dbg.onDebuggerStatement = function (frame) {

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

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

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

             s.setBreakpoint(offs[j], {hit: function () { g.log += "" + i; }});

     };

 }

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

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

        "    log += 'a';\n" +    // line0 + 2

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

        "}\n");

 for (var i = 0; i < 3; i++)

     attach(g, i);

 g.log = '';

 g.eval('debugger;');

 g.log += 'x';

 g.f();

 assertEq(g.log, 'xa012b');