The `reject` is not emitted for exception in the function called for resolving, so except some cases (like `done, assert.fail`), it's always better add an additional `then` in case the resolved function throw an error, otherwise it would be eaten.

Also, it seems that during the devtools work week the shortcut `function() statement`, that is Mozilla specific, was decided to be avoided (in favor of arrow function when is possible). It seems this is applying not only to devtools code but for jetpack too. We can ask to @Gozala for a confirm.