Fix for 'Concurrent modification during iteration' during search.

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

Index: pkg/analyzer/lib/src/dart/analysis/file_state.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/file_state.dart b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
index 542d032164269820601aca47ae91026e2bd365e8..f92cc1cd155dac300eaf562465bed2bba12bd088 100644
--- a/pkg/analyzer/lib/src/dart/analysis/file_state.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/file_state.dart
@@ -575,8 +575,8 @@ class FileSystemState {
   /**
    * Return the known files.
    */
-  Iterable<FileState> get knownFiles =>
-      _pathToFiles.values.map((files) => files.first);
+  List<FileState> get knownFiles =>
+      _pathToFiles.values.map((files) => files.first).toList();
 
   @visibleForTesting
   FileSystemStateTestView get test => _testView;