Fix JS generator to pass instanceOf test.
Fix JS generator to pass instanceOf test.

syohex/libbun-snapshot,syohex/libbun-snapshot,syohex/libbun-snapshot,syohex/libbun-snapshot,syohex/libbun-snapshot,syohex/libbun-snapshot,syohex/libbun-snapshot,syohex/libbun-snapshot