remove unused field from private class (dart-lang/yaml#60)

pkgs/yaml/lib/src/parser.dart

diff --git a/pkgs/yaml/lib/src/parser.dart b/pkgs/yaml/lib/src/parser.dart
index 4beb1de..fd24f88 100644
--- a/pkgs/yaml/lib/src/parser.dart
+++ b/pkgs/yaml/lib/src/parser.dart
@@ -712,9 +712,6 @@
   /// Expect [TokenType.STREAM_START].
   static const STREAM_START = _State('STREAM_START');
 
-  /// Expect the beginning of an implicit document.
-  static const IMPLICIT_DOCUMENT_START = _State('IMPLICIT_DOCUMENT_START');
-
   /// Expect [TokenType.DOCUMENT_START].
   static const DOCUMENT_START = _State('DOCUMENT_START');