Int64 is signed, but we use it to store unsigned 64-bit values

We are storing both signed and unsigned 64-bit integers in as instances of Int64, even though the Int64 class is intended for signed values. It's up to the caller to treat the the value appropriately.

This is confusing because methods on Int64 such as isNegative() return the wrong thing for numbers from 1<<31 to the top of the range.

To fix this we would need to add another type to the fixnum library:
https://github.com/dart-lang/core/issues/148