config.trashcan will be undefined for apps other than flappy.  In BlocklyApps.inject (which we call immediately after this), it has a default value of true.  BlocklyApps.inject does a merge of the default values and the provided values.  We can't just pass it config.trashcan, because that merge would cause us to override the default of true with undefined when config.trashcan is undefined.

One could argue that the property merge code should take the default value when the provided value is undefined, but that's a change that has a higher risk of breaking something else.