Add an AST factory method for fieldDeclaration 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 624f84005479d3e522ce24a8c4983dc9e4e342dd..0c65f34f43dde396d0468335f0cf4fb3c4ccd347 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -2765,6 +2765,11 @@ abstract class FieldDeclaration extends ClassMember {
   Token get covariantKeyword;
 
   /**
+   * Set the token for the 'covariant' keyword to the given [token].
+   */
+  void set covariantKeyword(Token token);
+
+  /**
    * Return the fields being declared.
    */
   VariableDeclarationList get fields;