The Tor Browser / file revision

 // Test interactions between ES6 generators and not-yet-standard

 // features.

 function assertSyntaxError(str) {

     var msg;

     var evil = eval;

     try {

         // Non-direct eval.

         evil(str);

     } catch (exc) {

         if (exc instanceof SyntaxError)

             return;

         msg = "Assertion failed: expected SyntaxError, got " + exc;

     }

     if (msg === undefined)

         msg = "Assertion failed: expected SyntaxError, but no exception thrown";

     throw new Error(msg + " - " + str);

 }

 // Destructuring binding.

 assertSyntaxError("function* f(x = yield) {}");

 assertSyntaxError("function* f(x = yield 17) {}");

 assertSyntaxError("function* f([yield]) {}");

 assertSyntaxError("function* f({ yield }) {}");

 assertSyntaxError("function* f(...yield) {}");

 // For each.

 assertSyntaxError("for yield");

 assertSyntaxError("for yield (;;) {}");

 assertSyntaxError("for yield (x of y) {}");

 assertSyntaxError("for yield (var i in o) {}");

 // Expression bodies.

 assertSyntaxError("function* f() yield 7");