Add base64-js to devDependencies

[luixxiul]

Fix #8712

Auditors:

Test Plan:

1. rm -rf node_modules
2. npm i
3. npm run watch

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

[bbondy]

not sure why this is needed, but given the error above ++