extract TypeMemberContributor from prefixed element contributor

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.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 services.completion.dart.manager;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart'
@@ skipping 47 matching lines @@
  * The information about a requested list of completions within a Dart file.
  */
 class DartCompletionRequestImpl extends CompletionRequestImpl
     implements DartCompletionRequest {
   /**
    * The cached completion target or `null` if not computed yet.
    */
   CompletionTarget _target;
 
   /**
+   * The [DartType] for Object in dart:core
+   */
+  InterfaceType _objectType;
+
+  /**
    * `true` if [resolveDeclarationsInScope] has partially resolved the unit
    * referenced by [target], else `false`.
    */
   bool _haveResolveDeclarationsInScope = false;
 
   /**
    * Initialize a newly created completion request based on the given request.
    */
   factory DartCompletionRequestImpl.forRequest(CompletionRequest request) {
     return new DartCompletionRequestImpl._(
@@ skipping 19 matching lines @@
     if (unit != null) {
       CompilationUnitElement elem = unit.element;
       if (elem != null) {
         return elem.library;
       }
     }
     return null;
   }
 
   @override
+  InterfaceType get objectType {
+    if (_objectType == null) {
+      Source coreUri = context.sourceFactory.forUri('dart:core');
+      LibraryElement coreLib = context.getLibraryElement(coreUri);
+      _objectType = coreLib.getType('Object').type;
+    }
+    return _objectType;
+  }
+
+  @override
   CompletionTarget get target {
     if (_target == null) {
       CompilationUnit unit = context.computeResult(source, PARSED_UNIT);
       _target = new CompletionTarget.forOffset(unit, offset);
     }
     return _target;
   }
 
   @override
   Future<CompilationUnit> resolveDeclarationsInScope() async {
@@ skipping 69 matching lines @@
     // Gracefully degrade if unit cannot be resolved
     if (resolvedUnit == null) {
       return;
     }
 
     // Recompute the target for the newly resolved unit
     _target = new CompletionTarget.forOffset(resolvedUnit, offset);
     _haveResolveDeclarationsInScope = true;
   }
 }