convert ImportedReferenceContributor to new API

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 cbd3a3f51e5c5a8c30234e439987e5f13ecb608e..c6a9a7489ba9a93dce50edd61535dfbb3febc657 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
@@ -63,6 +63,11 @@ abstract class DartCompletionContributor {
  */
 abstract class DartCompletionRequest extends CompletionRequest {
   /**
+   * Return the dart:core library element
+   */
+  LibraryElement get coreLib;
+
+  /**
    * Return the expression to the right of the "dot" or "dot dot",
    * or `null` if this is not a "dot" completion (e.g. `foo.b`).
    */