refactor Result class, remove asOk and asError (#2542)

As discussed in the PR for the Result pattern implementation
(https://github.com/flutter/website/pull/11444) @parlough recommended
that `asError` and `asOk` should be not be used, and instead we should
use proper exhaustiveness checking.

This PR removes the two "convenience" methods and refactors code.

In some cases, it was enough with writing a proper `if` clause, while in
others it was necessary to use a `switch`.

Still, they are present in the `testing` folder, as they can be useful
for testing purposes.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

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compass_app/app/lib/ui/results/view_models/results_viewmodel.dart

@@ -50,14 +50,16 @@ class ResultsViewModel extends ChangeNotifier {
   Future<Result<void>> _search() async {
     // Load current itinerary config
     final resultConfig = await _itineraryConfigRepository.getItineraryConfig();
-    if (resultConfig is Error) {
-      _log.warning(
-        'Failed to load stored ItineraryConfig',
-        resultConfig.asError.error,
-      );
-      return resultConfig;
+    switch (resultConfig) {
+      case Error<ItineraryConfig>():
+        _log.warning(
+          'Failed to load stored ItineraryConfig',
+          resultConfig.error,
+        );
+        return resultConfig;
+      case Ok<ItineraryConfig>():
     }
-    _itineraryConfig = resultConfig.asOk.value;
+    _itineraryConfig = resultConfig.value;
     notifyListeners();
 
     final result = await _destinationRepository.getDestinations();
@@ -86,15 +88,17 @@ class ResultsViewModel extends ChangeNotifier {
     assert(destinationRef.isNotEmpty, "destinationRef should not be empty");
 
     final resultConfig = await _itineraryConfigRepository.getItineraryConfig();
-    if (resultConfig is Error) {
-      _log.warning(
-        'Failed to load stored ItineraryConfig',
-        resultConfig.asError.error,
-      );
-      return resultConfig;
+    switch (resultConfig) {
+      case Error<ItineraryConfig>():
+        _log.warning(
+          'Failed to load stored ItineraryConfig',
+          resultConfig.error,
+        );
+        return resultConfig;
+      case Ok<ItineraryConfig>():
     }
 
-    final itineraryConfig = resultConfig.asOk.value;
+    final itineraryConfig = resultConfig.value;
     final result = await _itineraryConfigRepository
         .setItineraryConfig(itineraryConfig.copyWith(
       destination: destinationRef,
@@ -103,7 +107,7 @@ class ResultsViewModel extends ChangeNotifier {
     if (result is Error) {
       _log.warning(
         'Failed to store ItineraryConfig',
-        result.asError.error,
+        result.error,
       );
     }
     return result;