Pull the 'keyword' property into Token.

pkg/analyzer/lib/src/dart/ast/token.dart

Index: pkg/analyzer/lib/src/dart/ast/token.dart
diff --git a/pkg/analyzer/lib/src/dart/ast/token.dart b/pkg/analyzer/lib/src/dart/ast/token.dart
index 1791990e53cfb54a65be3b44962e2eea0281a9e5..39a3ab5954637d1ef912b4e82d7e86f053d9480d 100644
--- a/pkg/analyzer/lib/src/dart/ast/token.dart
+++ b/pkg/analyzer/lib/src/dart/ast/token.dart
@@ -142,9 +142,7 @@ class DocumentationCommentToken extends CommentToken {
  * A token representing a keyword in the language.
  */
 class KeywordToken extends SimpleToken {
-  /**
-   * The keyword being represented by this token.
-   */
+  @override
   final Keyword keyword;
 
   /**
@@ -253,6 +251,9 @@ class SimpleToken implements Token {
   bool get isUserDefinableOperator => type.isUserDefinableOperator;
 
   @override
+  Keyword get keyword => null;
+
+  @override
   int get length => lexeme.length;
 
   @override