fix spelling in core libraries

sdk/lib/io/file_system_entity.dart

Index: sdk/lib/io/file_system_entity.dart
diff --git a/sdk/lib/io/file_system_entity.dart b/sdk/lib/io/file_system_entity.dart
index e0b22daae66f9c279504efb96171c00dec328269..41bee01181a3c5afc143bb17973bf93fc91f7b2c 100644
--- a/sdk/lib/io/file_system_entity.dart
+++ b/sdk/lib/io/file_system_entity.dart
@@ -816,7 +816,7 @@ class FileSystemDeleteEvent extends FileSystemEvent {
  */
 class FileSystemMoveEvent extends FileSystemEvent {
   /**
-   * If the underlaying implementation is able to identify the destination of
+   * If the underlying implementation is able to identify the destination of
    * the moved file, [destination] will be set. Otherwise, it will be `null`.
    */
   final String destination;