Currently the `WidgetAnnotationElement._getKeyModifier` method will always be falsy on Linux, which seems like a simple oversight. Looking at all the other `FeatureTest.platform` accesses we only handle the `isMac`-case specially, and it seems reasonable to do the same thing here.
The reason that this hasn't led to any bug reports is most likely that the `modifier`-property seems completely unused in the scripting-implementation.
Finally, with these changes we can (slightly) simplify the `FeatureTest.platform` implementation.
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