add previous token field

pkg/front_end/lib/src/fasta/scanner/token.dart

Index: pkg/front_end/lib/src/fasta/scanner/token.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/token.dart b/pkg/front_end/lib/src/fasta/scanner/token.dart
index 0c29f1c6ad0baadb7181d53042fa36e4634611a8..43eed184995fa4a18c97672fad43a7488df9304e 100644
--- a/pkg/front_end/lib/src/fasta/scanner/token.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/token.dart
@@ -29,6 +29,15 @@ abstract class Token {
   Token next;
 
   /**
+   * The previous token in the token stream.
+   *
+   * Deprecated :: This exists for compatibility with the Analyzer token stream
+   * and will be removed at some future date.
+   */
+  @deprecated
+  Token previousToken;
+
+  /**
    * Return the first comment in the list of comments that precede this token,
    * or `null` if there are no comments preceding this token. Additional
    * comments can be reached by following the token stream using [next] until