W3C Touch events specification defines 4 accessors to get modifier keys states (altKey, ctrlKey, metaKey, shiftKey).

Those accessors are not exposed in https://github.com/flutter/engine/blob/28dce2beb1c4c1088048378289d17ed7fabcc0ad/lib/web_ui/lib/src/engine/dom.dart#L1144