You can't create masks inside Animator at this point, but here are a couple workarounds:

Workaround 1

You can simulate a mask by creating a full-stage layer that's the same color as your expected (host codebase, embedding) background, but with a hole punched out. You can then animate elements underneath, executing a mask.   

This doesn't work for every scenario, but sometimes it's good enough. 

Here are an example you can fork and play with

Workaround 2

You can create your mask in Sketch and mark the group that contains the mask as exportable, and it should reflect correctly in Animator.

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