Implement SearchEngine for the new driver.

pkg/analyzer/lib/src/dart/analysis/search.dart

Index: pkg/analyzer/lib/src/dart/analysis/search.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/search.dart b/pkg/analyzer/lib/src/dart/analysis/search.dart
index 1dfe7b3600bfc871370795e51bb8a8d6bc4a4cd4..3bdcdd17b10ccf1ac023c837c8c909c4153f5d1e 100644
--- a/pkg/analyzer/lib/src/dart/analysis/search.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/search.dart
@@ -39,6 +39,9 @@ class Search {
   Future<List<SearchResult>> _searchReferences_Local(
       Element element, bool isRootNode(AstNode n)) async {
     String path = element.source.fullName;
+    if (!_driver.addedFiles.contains(path)) {
+      return const <SearchResult>[];
+    }
 
     // Prepare the unit.
     AnalysisResult analysisResult = await _driver.getResult(path);