Fix `external` declaration in rollup config
Fix `external` declaration in rollup config

Fixes #759.

ternjs/acorn,mysticatea/acorn,TimothyGu/acorn,marijnh/acorn,ternjs/acorn,marijnh/acorn,ternjs/acorn,mysticatea/acorn,TimothyGu/acorn,TimothyGu/acorn,marijnh/acorn,mysticatea/acorn