extract TypeMemberContributor 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 b61b31b749a49248248247f93bae2a54c4e22751..cf3b798edfe1619a10e80e924e7795e39a8bd4c5 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
@@ -71,6 +71,11 @@ abstract class DartCompletionRequest extends CompletionRequest {
   Future<LibraryElement> get libraryElement;
 
   /**
+   * Answer the [DartType] for Object in dart:core
+   */
+  DartType get objectType;
+
+  /**
    * Return the completion target.  This determines what part of the parse tree
    * will receive the newly inserted text.
    */