Don't search if the driver cannot reference the element.

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 98087e0a854fec548608e20aef4c37c49a3f3d05..18114bd56a7df5b07597c5a1278161756c8ca5b2 100644
--- a/pkg/analyzer/lib/src/dart/analysis/search.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/search.dart
@@ -70,13 +70,19 @@ class Search {
 
   Future<Null> _addResults(List<SearchResult> results, Element element,
       IndexRelationKind relationKind, SearchResultKind resultKind) async {
+    String path = element.source.fullName;
+
+    // If the file with the element is not know, then the element is not used.

[Paul Berry, 2016/11/28 21:45:10]

s/know/known/

+    if (!_driver.knownFiles.contains(path)) {
+      return;
+    }
+
     // TODO(scheglov) optimize for private elements
     String name = element.displayName;
 
     // Prepare the list of files that reference the element name.
     List<String> files = await _driver.getFilesReferencingName(name);
-    String path = element.source.fullName;
-    if (!files.contains(path)) {
+    if (!files.contains(path) && _driver.addedFiles.contains(path)) {
       files.add(path);
     }