Previously, applying:

SchemaDirectiveVisitor.visitSchemaDirectives(federatedSchema, schemaDirectives);

Would allow custom directives to be applied to a federated schema.

In 5.0, this fails with an exception:

TypeError: Cannot read property 'concat' of undefined

Due to this line:

directiveNodes = directiveNodes.concat(extensionASTNode.directives);

In this case, directiveNodes is undefined because for the Extension type, type.astNode is not undefined but type.astNode.directives is.