extract TypeMemberContributor from prefixed element contributor

pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart

Index: pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
diff --git a/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart b/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
index c1949294a62a8d9315578dddeef60f2bf124cb0e..3ecbd3eecc4e478c5dcc5ef8dfb1140a87d60457 100644
--- a/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
+++ b/pkg/analysis_server/lib/src/services/completion/dart/completion_manager.dart
@@ -65,6 +65,11 @@ class DartCompletionRequestImpl extends CompletionRequestImpl
   CompletionTarget _target;
 
   /**
+   * The [DartType] for Object in dart:core
+   */
+  InterfaceType _objectType;
+
+  /**
    * `true` if [resolveDeclarationsInScope] has partially resolved the unit
    * referenced by [target], else `false`.
    */
@@ -104,6 +109,16 @@ class DartCompletionRequestImpl extends CompletionRequestImpl
   }
 
   @override
+  InterfaceType get objectType {
+    if (_objectType == null) {
+      Source coreUri = context.sourceFactory.forUri('dart:core');
+      LibraryElement coreLib = context.getLibraryElement(coreUri);
+      _objectType = coreLib.getType('Object').type;
+    }
+    return _objectType;
+  }
+
+  @override
   CompletionTarget get target {
     if (_target == null) {
       CompilationUnit unit = context.computeResult(source, PARSED_UNIT);