Another way of fixing it would be, whichever character caused 'StringUTF16.compress' to fail (we'd need to return it), make sure that character is kept in the final string (could just assign it at the end, remember it's value and location earlier).
By merging in the code of StringUTF16.compress and StringUTF16.toBytes into one function (and they are both very small), this wouldn't even have to slow things down, once you noticed location 'i' has char 'c' which isn't LATIN1, you make the new buffer, copy 0..i-1 over, then your known non-LATIN1 'i', then i+1..len.
This might be very slightly slower, but would fix (I think) the problem of breaking interning, which feels quite painful.
By merging in the code of StringUTF16.compress and StringUTF16.toBytes into one function (and they are both very small), this wouldn't even have to slow things down, once you noticed location 'i' has char 'c' which isn't LATIN1, you make the new buffer, copy 0..i-1 over, then your known non-LATIN1 'i', then i+1..len.
This might be very slightly slower, but would fix (I think) the problem of breaking interning, which feels quite painful.