Upstreaming from https://phabricator.wikimedia.org/T227255 (which exposes a slightly different behavior)

Steps to reproduce:

• Have more than 100 Maniphest tasks which you have flagged in https://secure.phabricator.com/
• Navigate to https://secure.phabricator.com/flag/
• Click the Next button at the bottom.

Actual Results:

• Nothing happens in Firefox 67.

Expected Results:

• The next page shows.

Additional steps, to continue from above:

• Add an arbitrary ?after=### URL parameter.

Actual Results:

• You get the following error:

Call to undefined method PhabricatorFlagQuery::withIDs()

Reproducibility:
Always

Version information:
This very instance secure.phabricator.com at this very time.