extract LibraryMemberContributor from prefixed element contributor

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

Index: pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
diff --git a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
index faf56be56b57495ebfb7eba394d4483d3d62697c..b61b31b749a49248248247f93bae2a54c4e22751 100644
--- a/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
+++ b/pkg/analysis_server/lib/src/provisional/completion/dart/completion_dart.dart
@@ -88,10 +88,11 @@ abstract class DartCompletionRequest extends CompletionRequest {
 
   /**
    * Return a [Future] that completes when the element associated with
-   * the given [identifier] is available or if the identifier cannot be resolved
+   * 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 resolveIdentifier(SimpleIdentifier identifier);
+  Future resolveExpression(Expression expression);
 }