migration.js is a special case where we generally need blocking
calls in order for the wizard to work correctly. Accordingly we
block waiting for the new async interfaces. With automigration
and potential new UIs that are in the works for migration, the
asynchronicity of these interfaces will be more relevant, but
here it's not really important enough to make big changes to the
way the UI is implemented.
MozReview-Commit-ID: LkfwBVfpCJO
73907ee665