do not suggest completions for cascade on library prefix - fixes #25215

pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analysis_server.src.provisional.completion.completion_dart;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart';
@@ skipping 43 matching lines @@
       DartCompletionRequest request);
 }
 
 /**
  * The information about a requested list of completions within a Dart file.
  *
  * Clients may not extend, implement or mix-in this class.
  */
 abstract class DartCompletionRequest extends CompletionRequest {
   /**
+   * Return the expression to the right of the "dot" or "dot dot",
+   * or `null` if this is not a "dot" completion (e.g. `foo.b`).
+   */
+  Expression get dotTarget;
+
+  /**
    * Return a [Future] that completes with the library element
    * which contains the unit in which the completion is occurring.
    * The [Future] may return `null` if the library cannot be determined
    * (e.g. unlinked part file).
    * Any information obtained from [target] prior to calling this method
    * should be discarded as it may have changed.
    */
   Future<LibraryElement> get libraryElement;
 
   /**
@@ skipping 30 matching lines @@
   /**
    * Return a [Future] that completes when the element associated with
    * the given [expression] in the target compilation unit is available.
    * It may also complete if the expression cannot be resolved
    * (e.g. unknown identifier, completion aborted, etc).
    * Any information obtained from [target] prior to calling this method
    * should be discarded as it may have changed.
    */
   Future resolveExpression(Expression expression);
 }