In Meteor 1.2, subclassing a class defined in a package throws an error.
Here we define classes A and B in package "class-a", then try to subclass them in the app. The file client/class.js defines subclasses C and D of A and B, respectively. The classes are instantiated in the file client/es6-errors-reproduction.js.
I tried a few different ways of doing this, but all of them throw this error: TypeError: Class constructors cannot be invoked without 'new'.
Note that this is not the error that gets thrown if you forget to put 'new' in front of a constructor call. That tends to throw: TypeError: Cannot call class as a function.
To see the errors, run this app and open the JavaScript console.