Because GWT code runs in an iframe,
JsPackage.GLOBAL maps to the JSNI
$wnd equivalent referencing the
parent window so anything referenced from JsInterop needs to be recorded as a
window property; and classes aren't recorded on the
window per ECMAScript (see
https://stackoverflow.com/a/37711826/116472), this is why you need to do it explicitly to expose them to JsInterop.