Hi Eliot, it looks like any attempt to position a file beyond some
threshold is causing the primitive failure in Cog VM's released after
2776 that I'm seeing.

| file | file:=StandardFileStream newFileNamed: 'mybigfile'.  file binary.
[1234 timesRepeat: [ file nextPutAll: (ByteArray new: 1 million) ].
file position: file size - 100 ]
    ensure: [file close].

Magma uses a random file position for every single object retrieved;
so the performance of primitiveFileSetPosition is crucial to the
performance of Magma.

Thanks.