extract named constructor suggestions 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 816894d639106146447ccdb805d7928ecb3b8e0d..ad22f37e834e6f4afbf11de4f9869f3d1a2fb746 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
@@ -10,6 +10,7 @@ import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/provisional/completion/completion_core.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion_target.dart';
 import 'package:analyzer/src/generated/ast.dart';
+import 'package:analyzer/src/generated/element.dart';
 
 export 'package:analysis_server/src/provisional/completion/completion_core.dart'
     show EMPTY_LIST;
@@ -66,6 +67,16 @@ abstract class DartCompletionRequest extends CompletionRequest {
   CompletionTarget get target;
 
   /**
+   * Return a [Future] that completes with the library element
+   * which contains the unit in which the completion is occurring.
+   * The [Future] may return `null` if the library cannot be resolved
+   * (e.g. unlinked part file).
+   * Any information obtained from [target] prior to calling this method
+   * should be discarded as it may have changed.
+   */
+  Future<LibraryElement> resolveContainingLibrary();
+
+  /**
    * Return a [Future] that completes with a compilation unit in which
    * all declarations in all scopes containing [target] have been resolved.
    * The [Future] may return `null` if the unit cannot be resolved