The Tor Browser / file revision

 // Test the corner case of accessing an unaliased variable of a block

 // while the block is not live.

 var g = newGlobal();

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

 g.eval("function f() { let (x = 1, y) { (function() { y = 0 })(); h() } }");

 g.eval("var surprise = null");

 var dbg = new Debugger(g);

 dbg.onDebuggerStatement = function(hFrame) {

     var fFrame = hFrame.older;

     assertEq(fFrame.environment.getVariable('x'), 1);

     assertEq(fFrame.environment.getVariable('y'), 0);

     fFrame.eval("surprise = function() { return ++x }");

     assertEq(g.surprise(), 2);

 }

 g.f();

 assertEq(g.surprise !== null, true);

 // Either succeed or throw an error about 'x' not being live

 try {

     assertEq(g.surprise(), 3);

 } catch (e) {

     assertEq(e+'', 'Error: x is not live');

 }