Add AST factories for formal parameters with all optional named parameters.

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 38e477fc02b1d52ebad44d685481a8acb3826d63..624f84005479d3e522ce24a8c4983dc9e4e342dd 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -5311,6 +5311,11 @@ abstract class NodeList<E extends AstNode> implements List<E> {
  */
 abstract class NormalFormalParameter extends FormalParameter {
   /**
+   * Set the token for the 'covariant' keyword to the given [token].
+   */
+  void set covariantKeyword(Token token);
+
+  /**
    * Return the documentation comment associated with this parameter, or `null`
    * if this parameter does not have a documentation comment associated with it.
    */