We "import" modules by executing them and taking their global scope as the
module object. In ES6, variables declared with "let" and "const" do not end
up in the global scope any longer. Instead, they end up in the "global
lexical scope", which is a different object. Unfortunately, this means
breaking the way many modules export their variables, but if you want
a symbol to be exported, you have to declare it with "var" or place it
explicitly on the global object some other way.

https://bugzilla.gnome.org/show_bug.cgi?id=781429