Freeze the global value properties: NaN, Infinity, undefined.

Test that these properties are not writable, enumerable, configurable.
Combine all global object tests into one suite.
Remove tests for Global object internal properties.

These are defined in spec as implementation-dependant.
Evaluate compound assignement expressions.

Everything, except the OpURShiftNode, because Ruby doesn't have >>>
operator built in.
Add builtin JS::Error classes.

To be raised as exceptions when evaluating JS code.
Implement evaluation of 'in' operator.
Implement JS::Base#enumerable_keys.
Initial for-in implementation.