v3.3.0 — Covering the untested subsystems

Covering the untested subsystems, and the bugs that were hiding in them. Test
coverage went from 58% to 67%; the files with no coverage at all went from 17
to 5, all of which are backed by platform views.

Fixed

• A form beginning with a QuestionStep never reported its results.
  Reaching the end reset the form to its first step before firing the
  completion callback — and rendering a step reassigns onFinish from that
  step, so a leading question (carrying no callback of its own) replaced it
  with null. onFinish silently never ran and the submission was lost. The
  callback is now captured before navigating, and receives a copy of the
  answers rather than the map that is about to be cleared.
• The completion callback fired twice when no condition matched. The
  unmatched-condition path reported, then fell through and reported again —
  the second time after the answers had been cleared.
• The review step listed every step, including itself. Instructions, the
  completion step and the review step all appeared as "Not provided", because
  the flat result map carries an entry per step rather than per answer.
• JSON-defined steps lost their button labels. An absent nextButtonText
  passed null, wiping the per-type default: a JSON DisplayStep showed "Next"
  where a Dart one showed "Start", a ReviewStep "Next" instead of "Submit",
  a CompletionStep "Next" instead of "Finish".
• An unknown enum value in JSON threw a bare StateError. display,
  selectionType, componentsStyle, inputStyle and displayStepType now
  report the field, the value and the permitted set.
• DynamicConditionalRelevant crashed on a null callback. The parameter is
  nullable but was force-unwrapped, so a condition built without one threw
  during navigation instead of simply not matching.
• PopStep threw at the root of an app, where there is nothing to pop, and
  could touch a defunct context if the view was disposed before its frame.

Added

• PopStep is exported. It was usable from JSON as {"type": "PopStep"} but
  unreachable from Dart.
• Tests for the previously untested subsystems: the expression evaluators,
  FormStackLocale, StaticDataProvider, DynamicConditionalRelevant, the
  consent, review, completion, display and pop step views, and the geotrace
  coordinate input.

Notes

Five files remain without coverage — the Google Maps widgets, the map input,
the web view and the HTML editor. All are backed by platform views, which a
widget test cannot instantiate; they need an integration test on a device.