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>

pkg/analysis_server/tool/spec/generated/java/AnalysisServer.java

@@ -778,17 +778,6 @@ public interface AnalysisServer {
    */
   public void execution_setSubscriptions(List<String> subscriptions);
 
-  /**
-   * {@code flutter.getChangeAddForDesignTimeConstructor}
-   *
-   * Return the change that adds the forDesignTime() constructor for the widget class at the given
-   * offset.
-   *
-   * @param file The file containing the code of the class.
-   * @param offset The offset of the class in the code.
-   */
-  public void flutter_getChangeAddForDesignTimeConstructor(String file, int offset, GetChangeAddForDesignTimeConstructorConsumer consumer);
-
   /**
    * {@code flutter.setSubscriptions}
    *