app/node_modules/@angular/material/form-field/testing/control/index.d.ts

import { ComponentHarness } from '@angular/cdk/testing';

/**
 * Base class for custom form-field control harnesses. Harnesses for
 * custom controls with form-fields need to implement this interface.
 */
declare abstract class MatFormFieldControlHarness extends ComponentHarness {
}

export { MatFormFieldControlHarness };