The Tor Browser / file revision

 // Enabled debuggers keep breakpoint handlers alive.

 var g = newGlobal();

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

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

        "    return 2;\n" +      // line0 + 2

        "}\n");

 var N = 4;

 var hits = 0;

 for (var i = 0; i < N; i++) {

     var dbg = Debugger(g);

     dbg.onDebuggerStatement = function (frame) {

         var handler = {hit: function () { hits++; }};

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

         var offs = s.getLineOffsets(g.line0 + 2);

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

             s.setBreakpoint(offs[j], handler);

     };

 }

 g.eval('debugger;');

 gc({});

 assertEq(g.f(), 2);

 assertEq(hits, N);