This change does not fix any bug, but the SMJS builtin classes use
negative tinyids already, so I presume this is the preferred practice.
At least it means the tinyids won't have to be renumbered later if
some of these objects are changed to behave as arrays.
If ECMAScript code does obj[42], then the getProperty or setProperty
function of the JSClass of obj gets 42 as the property ID and must not
treat that as an internal error.
