Ast changes to support the covariant keyword

pkg/analyzer/lib/dart/ast/ast.dart

Index: pkg/analyzer/lib/dart/ast/ast.dart
diff --git a/pkg/analyzer/lib/dart/ast/ast.dart b/pkg/analyzer/lib/dart/ast/ast.dart
index cb171d8457e8d17c097c689b77e31db7510f5a97..9024c486fd22ca1aef06f433762781e21cb7197a 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -2991,6 +2991,11 @@ abstract class ForEachStatement extends Statement {
  */
 abstract class FormalParameter extends AstNode {
   /**
+   * The 'covariant' keyword, or `null` if the keyword was not used.
+   */
+  Token get covariantKeyword;
+
+  /**
    * Return the element representing this parameter, or `null` if this parameter
    * has not been resolved.
    */