Fix unlikely but possible race in `WeakValueDictionary`
It may happen that value in `valueArray` could have been already
collected by the GC but #clearDeadSlots have not yet been called.
When this happened, `#at:ifAbsentPut:` returned tombstone rather
than updating the dictionary with value from block.
This commit fixes this by checking whether `valueArray` contain
the tombstone and if so, clearing up the dead slots and restarting
the operation. HTH.
@REM -------
@REM make using lcc compiler
@REM type lccmake, and wait...
@REM do not edit - automatically generated from ProjectDefinition
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make.exe -N -f bc.mak -DUSELCC=1 %*