typescript: make code compatible with typescript 2.4
typescript: make code compatible with typescript 2.4

Summary:
Forward-declare a type found in TypeScript 2.4 to make this
code work both in TS2.3 and TS2.4.

Reviewers: #core_team, fromberger

Reviewed By: #core_team, fromberger

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1828

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