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.