Fatal Error after upgrading from 5.4.2 to 5.5.0

As @bschneiders mentioned, this seems to be related to how we're exporting module files in 5.5, which requires some updates to custom plugins before they are compatible with 5.5.0.

I put some details about this change in a comment on this issue: https://github.com/elastic/kibana/issues/12707#issuecomment-314216338