The Tor Browser / file revision

 // setVariable cannot modify the binding for a FunctionExpression's name.

 load(libdir + "asserts.js");

 var g = newGlobal();

 var dbg = Debugger(g);

 var hits = 0;

 dbg.onDebuggerStatement = function (frame) {

     var env = frame.environment.find("f");

     assertEq(env.getVariable("f"), frame.callee);

     assertThrowsInstanceOf(function () { env.setVariable("f", 0) }, TypeError);

     assertThrowsInstanceOf(function () { env.setVariable("f", frame.callee) }, TypeError);

     hits++;

 };

 g.eval("(function f() { debugger; })();");

 assertEq(hits, 1);