Migrator: Land front-end support for 'migration status' check boxes

These check boxes are hidden behind a parameter that must be changed during
development; it is not wired to a public-facing flag.

The check boxes are styled, and are connected to the starting status of each
compilation unit (already migrated, opted out, migrating).

The check boxes can be toggled, but just for mocking purposes; their status is
not connected to the back end. The directory check boxes are not wired to the
status of their children.

Screenshot of checkboxes w/o colors: https://screenshot.googleplex.com/43XBRWsMPgbQXik

Screenshot of check boxes w/ colors: https://screenshot.googleplex.com/3bhJqcnmeV4sGNm
* dark green - already migrated; file is not being changed.
* light green - file is being migrated now.
* orange un-checked - file is being opted out, initial migration,
  file will gain opt-out comment.
* grey un-checked - staying opted out, file is not being changed.
* indeterminate - folder has multiple states inside.

Bug: https://github.com/dart-lang/sdk/issues/44124 and
Change-Id: I49daf1f52544024ec1c6d8d5cf3c59515c7db3a7
     https://github.com/dart-lang/sdk/issues/44101
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/172821
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>