extract LibraryMemberContributor 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 136 matching lines @@
       return null;
     }
 
     // Recompute the target for the newly resolved unit
     _target = new CompletionTarget.forOffset(resolvedUnit, offset);
     _haveResolveDeclarationsInScope = true;
     return resolvedUnit;
   }
 
   @override
-  Future resolveIdentifier(SimpleIdentifier identifier) async {
-    if (identifier.bestElement != null) {
-      return;
-    }
-
+  Future resolveExpression(Expression expression) async {
     //TODO(danrubel) resolve the expression or containing method
     // rather than the entire complilation unit
 
     CompilationUnit unit = target.unit;
 
     // Determine the library source
     Source librarySource;
     if (unit.directives.any((d) => d is PartOfDirective)) {
       List<Source> libraries = context.getLibrariesContaining(source);
       if (libraries.isEmpty) {
@@ skipping 18 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;
   }
 }