convert ImportedReferenceContributor to new API

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 45 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 dart:core library element
+   */
+  LibraryElement get coreLib;
+
+  /**
    * 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 `true` if free standing identifiers should be suggested
    */
   bool get includeIdentifiers;
 
@@ skipping 38 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);
 }