extract LibraryPrefixContributor from imported reference 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 b651ddb7e22f36cf81c3439c5e2d517e8ed117aa..827024b6d85f89513de5f30d75b30bdb73247106 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
@@ -68,6 +68,11 @@ abstract class DartCompletionRequest extends CompletionRequest {
   Expression get dotTarget;
 
   /**
+   * Return `true` if free standing identifiers should be suggested
+   */
+  bool get includeIdentifiers;
+
+  /**
    * Return the library element which contains the unit in which the completion
    * is occurring. This may return `null` if the library cannot be determined
    * (e.g. unlinked part file).