Just dropping this off for later thorough investigation:
Issue appears to be repeatable. Issue arises in instances where a hashmap key set is retrieved and then the size of the set is requested.
Exact cause is yet to be determined as to why the minified has this result.
Full minified is attached.
j4ts.min.zip

Minified j4ts.js included below. Error case is found at j4ts.min.js?v=167416913297991951691696567159:1:305201

c.prototype.remove = function(t) { return !!this.__parent.containsKey(t) && (this.remove(t), !0) } , c.prototype.size = function() { return this.size() } ; var s, e = c; function c(t) { var e = s.call(this) || this; return e.__parent = t, e } function h(t, e) { this.outerIter = e, this.__parent = t } (t.AbstractMap$0 = e).__interfaces = ["java.util.Collection", "java.util.Set", "java.lang.Iterable"],