update completion extension point to use CompletionContributorFactory

pkg/analysis_server/lib/src/provisional/completion/completion_core.dart

Index: pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
diff --git a/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart b/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
index d8567f79139e072333e85da77b89ba6669ccab46..bdebfcc0c61c84d83f95e707b01b148bc744d56f 100644
--- a/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
+++ b/pkg/analysis_server/lib/src/provisional/completion/completion_core.dart
@@ -19,6 +19,13 @@ import 'package:analyzer/src/generated/source.dart';
 const EMPTY_LIST = const <CompletionSuggestion>[];
 
 /**
+ * An object used to instantiate a [CompletionContributor] instance
+ * for each 'completion.getSuggestions' request.
+ * Contributors should *not* be cached between requests.
+ */
+typedef CompletionContributor CompletionContributorFactory();
+
+/**
  * An object used to produce completions at a specific location within a file.
  *
  * Clients may implement this class when implementing plugins.