Add SearchEngine.membersOfSubtypes().

pkg/analysis_server/lib/src/services/search/search_engine.dart

Index: pkg/analysis_server/lib/src/services/search/search_engine.dart
diff --git a/pkg/analysis_server/lib/src/services/search/search_engine.dart b/pkg/analysis_server/lib/src/services/search/search_engine.dart
index c2235a17b66036c8fce971ca9bac37511d03332f..df6e31c4d72ccd419b843d58bde9a862a36cb3bb 100644
--- a/pkg/analysis_server/lib/src/services/search/search_engine.dart
+++ b/pkg/analysis_server/lib/src/services/search/search_engine.dart
@@ -56,6 +56,13 @@ class MatchKind {
  */
 abstract class SearchEngine {
   /**
+   * If the [type] has subtypes, return the set of names of members which these
+   * subtypes declare, possibly empty.  If the [type] does not have subtypes,
+   * return `null`.
+   */
+  Future<Set<String>> membersOfSubtypes(ClassElement type);
+
+  /**
    * Returns all subtypes of the given [type].
    *
    * [type] - the [ClassElement] being subtyped by the found matches.