A shorthand property and an object literal passed as an argument no
longer widen the declared record layout of the binding or parameter
that receives them. A widened layout made every existing record that
flowed into that binding or parameter convert to a copy, so later
writes through the copy were lost. Those literals keep a missing
element in their own fields and convert, with a check, into a declared
record.
The debug-variables fixture iterates an array whose elements may be
undefined, so its loop binding keeps the union layout the debugger
check inspects.
Chris Tate