Don't compute 'dart:core' LibraryElement during completion.

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.
 
 import 'dart:async';
 
 import 'package:analysis_server/src/provisional/completion/completion_core.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 28 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 29 matching lines @@
   SourceFactory get sourceFactory;
 
   /**
    * Return the completion target.  This determines what part of the parse tree
    * will receive the newly inserted text.
    * At a minimum, all declarations in the completion scope in [target.unit]
    * will be resolved if they can be resolved.
    */
   CompletionTarget get target;
 }