See D14617. This could probably go either way but we don't currently need $engine in newStandardEditField(), so just get rid of it.
Edited a task with standard custom fields defined.
I think subclasses might reasonably want to override this method and need access to $engine to do so. This is theoretical in the upstream, but PhabricatorCustomField is extensible by third-party code, and we have some (non-custom) fields in first-party applications where behavior depends on $editor (for example, fields which only appear on either the "create" or "edit" screen, but not both).