set param(p) {
    this.__param = p;
    /**
     * This event is listened for by ValidateMixin. Whenever the validation parameter has
     * changed, the FormControl will revalidate itself
     */
    this.dispatchEvent(new Event('param-changed'));
  }
The first argument of the constructor, for instance 3 in `new MinLength(3)`. Will
be stored on Validator instance and passed to `execute` function
@example
```js
 // Store reference to Validator instance
 const myValidatorInstance = new MyValidator(1);
 // Use this instance initially on a FormControl (that uses ValidateMixin)
 render(html`<validatable-element .validators="${[myValidatorInstance]}"></validatable-element>`, document.body);
 // Based on some event, we need to change the param
 myValidatorInstance.param = 2;
```
@property {ValidatorParam}