SPRINGFIELD — Illinois did not have an estate tax law on the books between 2010 and 2011.That changed in early 2011, around the time state lawmakers approved an income tax hike on individuals and corporations. The new estate tax applied retroactively and was tied to the beginning of the calendar year.But a constitutional challenge suggests the tax, effective on Jan. 13, 2011, cannot be applied retroactively. And now the state’s top court could decide whether those claims have any merit.The Illinois Supreme …