function fromVLQSigned(aValue) {
  var isNegative = (aValue & 1) === 1;
  var shifted = aValue >> 1;
  return isNegative
    ? -shifted
    : shifted;
}
Converts to a two-complement value from a value where the sign bit is
placed in the least significant bit.  For example, as decimals:
  2 (10 binary) becomes 1, 3 (11 binary) becomes -1
  4 (100 binary) becomes 2, 5 (101 binary) becomes -2