Add missing AST support for covariant keyword and fix bug from previous CL

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 53522941d20bdd8e0ac1ebc2f8e7daf0910cdb20..38e477fc02b1d52ebad44d685481a8acb3826d63 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -2760,6 +2760,11 @@ abstract class ExtendsClause extends AstNode {
  */
 abstract class FieldDeclaration extends ClassMember {
   /**
+   * The 'covariant' keyword, or `null` if the keyword was not used.
+   */
+  Token get covariantKeyword;
+
+  /**
    * Return the fields being declared.
    */
   VariableDeclarationList get fields;