Consumer.php

<?php

namespace Foo\Bar\Whistle;

use Drupal\Core\RequestInfo;

class Sniffer {
  
  public function getMine($request) {
    $raw = RequestInfo::getRawVariables($request);
    return $raw->get('bar');
  } 
}

ConsumerTest.php

<?php

namespace Foo\Bar\Whistle\Tests;


use Foo\Bar\Whistle\Sniffer;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\ParameterBag;

class ConsumerTest {
  
  protected $sniffer;
  
  public function setUp() {
    $this->sniffer = new Sniffer();
  }
  
  public function testGetMine() {
    $request = new Request();
    $bag = new ParameterBag();
    $bag->set('bar', 'goat');
    $request->attributes->set('_raw_variables', $bag);
    $this->assertEquals($this->sniffer->getMine($request), 'goat');
    
  }
}

requestinfo.php

<?php
namespace Drupal\Core\Routing;

use Symfony\Component\HttpFoundation\Request;

class RequestInfo {

  // Gets the raw variables.
  public static method getRawVariables(Request $request) {
    return $request->attributes->get('_raw_variables');
  }
}