Graduate 'flutter' domain from experimental, remove unused methods and fields.

We could also remove FlutterCorrections, because right now we don't
use it, but we will restore it soon as we get new Flutter domain
methods.

R=brianwilkerson@google.com, devoncarew@google.com

Change-Id: I084fe5acbe5d854673c7247d734aeca1f8d721af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108364
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analysis_server/doc/api.html

@@ -276,20 +276,9 @@ const String EXECUTION_RESPONSE_MAP_URI_FILE = 'file';
 const String EXECUTION_RESPONSE_MAP_URI_URI = 'uri';
 const String FLUTTER_NOTIFICATION_OUTLINE = 'flutter.outline';
 const String FLUTTER_NOTIFICATION_OUTLINE_FILE = 'file';
-const String FLUTTER_NOTIFICATION_OUTLINE_INSTRUMENTED_CODE =
-    'instrumentedCode';
 const String FLUTTER_NOTIFICATION_OUTLINE_OUTLINE = 'outline';
-const String FLUTTER_REQUEST_GET_CHANGE_ADD_FOR_DESIGN_TIME_CONSTRUCTOR =
-    'flutter.getChangeAddForDesignTimeConstructor';
-const String FLUTTER_REQUEST_GET_CHANGE_ADD_FOR_DESIGN_TIME_CONSTRUCTOR_FILE =
-    'file';
-const String FLUTTER_REQUEST_GET_CHANGE_ADD_FOR_DESIGN_TIME_CONSTRUCTOR_OFFSET =
-    'offset';
 const String FLUTTER_REQUEST_SET_SUBSCRIPTIONS = 'flutter.setSubscriptions';
 const String FLUTTER_REQUEST_SET_SUBSCRIPTIONS_SUBSCRIPTIONS = 'subscriptions';
-const String
-    FLUTTER_RESPONSE_GET_CHANGE_ADD_FOR_DESIGN_TIME_CONSTRUCTOR_CHANGE =
-    'change';
 const String KYTHE_REQUEST_GET_KYTHE_ENTRIES = 'kythe.getKytheEntries';
 const String KYTHE_REQUEST_GET_KYTHE_ENTRIES_FILE = 'file';
 const String KYTHE_RESPONSE_GET_KYTHE_ENTRIES_ENTRIES = 'entries';