Fix compilerJs to generate proper JavaScript source code for overridden virtual fields:

@Js class SlotsA
{
  virtual Str a := "SlotsA"
}

@Js class SlotsB : SlotsA
{
  override final Str a := "SlotsB"
}

Promoted to ticket #1782 and assigned to andy

Ticket resolved in 1.0.62

Fixed - changeset