Should Types (e.g. Manifest, Sequence) raise warning when unexpected values are marshaled?

[jpstroop]

AbstractObject has ALLOWED_ANYWHERE_PROPS, Manifest has ARRAY_KEYS and (will have) ANY_TYPE_KEYS. If all types had these, we could check all of the keys and raise warnings when unexpected keys are present (they can always be added via []=). User could supply an argument (e.g. force=true) if they really want these to make it through.