Weird Type Mismatch error for JS backend (1.6.X)

[ElegantBeef]

The older versions 'incorrectly' allowed a procedure that took a child type to 
upcast into a procedure that takes a parent type. The solution is to either 
internally convert the parameter into the child
    
    
    let event = MouseEvent event
    
    
    Run

or cast the procedure to the base
    
    
    cast[proc(event: Event)](myProcHere)
    
    
    Run