Fall back to defaultFn when property present but undefined.

This relaxes type checking slightly to accommodate parser code that can't
help creating undefined properties when returning object literals.
Use ClassBodyElement to type the "body" field of ClassBody.
Improve XJS-related type definitions.

Added missing default functions and made a few disjunction (or) types more
liberal to accommodate the actual output of the fb-harmony parser.
Shorten file names in test output.

This is a purely cosmetic improvement.
Bump NPM version to 0.2.10 in preparation for publishing.