invariant is not defined

[pachewise]

Getting an "invariant is not defined" error in one of our production builds (while we were testing it). Looking at the stack trace, it led us to your library, which we just upgraded 2.1.0 -> 2.2.0.

Which led us to

react-immutable-proptypes/src/ImmutablePropTypes.js

Lines 40 to 43 in b2c415d

	invariant(
	false,
	'ImmutablePropTypes type checking code is stripped in production.'
	);

Is there a reason why invariant isn't imported here?