Fix a completion exception during import suggestions.

pkg/analyzer/lib/file_system/file_system.dart

Index: pkg/analyzer/lib/file_system/file_system.dart
diff --git a/pkg/analyzer/lib/file_system/file_system.dart b/pkg/analyzer/lib/file_system/file_system.dart
index 11223c92fa8992a7505798a0ab63060c53b33b4f..687380f3c4fc823901c1ab31fd92315f4bdd2746 100644
--- a/pkg/analyzer/lib/file_system/file_system.dart
+++ b/pkg/analyzer/lib/file_system/file_system.dart
@@ -150,6 +150,8 @@ abstract class Folder implements Resource {
   /**
    * Return a list of existing direct children [Resource]s (folders and files)
    * in this folder, in no particular order.
+   *
+   * On I/O errors, this will throw [FileSystemException].
    */
   List<Resource> getChildren();
 }