As a workaround you can probably copy GWT's Optional emulation into your own project, add JsInterop annotations and then it should work.--
GWT does not place JsInterop annotations on most JRE emulations because the compiler only has an "all-or-nothing" switch (-generatesJsInteropExports). That means all code that can be accessed from the @JsType annotated class can not be pruned because it might be called by external JS. So it would result in a final JS code size increase.
But you can make a feature request on Github, as GWT has @JsType some Collections: https://gwt-review.googlesource.com/#/c/15191/
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