WARNING: Server message without client id received

Vaadin 6 / 7 / 8
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Even after manually clearing Widgetset directories can see “var $gwt_version = "7.5.8"” in:

53CD41F41F6D8C3C83FB4A808EFD8C1A.cache.js
com.jgk.venn.frontend.vaadin.poc.PocWidgetSet.devmode.js

How is the $gwt_version set?