[analysis_server] Add a server.cancelRequest request for cancelling outstanding requests

Change-Id: I1b43b0e61e5cc89261170ce13613fad756600d97
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/222201
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -980,6 +980,20 @@ public interface AnalysisServer {
    */
   public void search_getTypeHierarchy(String file, int offset, boolean superOnly, GetTypeHierarchyConsumer consumer);
 
+  /**
+   * {@code server.cancelRequest}
+   *
+   * Requests cancellation of a request sent by the client by id. This is provided on a best-effort
+   * basis and there is no guarantee the server will be able to cancel any specific request. The
+   * server will still always produce a response to the request even in the case of cancellation, but
+   * clients should discard any results of any cancelled request because they may be incomplete or
+   * inaccurate. This request always completes without error regardless of whether the request is
+   * successfully cancelled.
+   *
+   * @param id The id of the request that should be cancelled.
+   */
+  public void server_cancelRequest(String id);
+
   /**
    * {@code server.getVersion}
    *