TestMemoryBackend example

->willReturn('en');
    $this->english = $english;
    $japanese = $this->createMock('\Drupal\Core\Language\LanguageInterface');
    $japanese->expects($this->any())
      ->method('getId')
      ->willReturn('jp');
    $this->japanese = $japanese;
    $this->languageManager = $this->createMock('\Drupal\Core\Language\LanguageManagerInterface');
    $this->languageManager->expects($this->any())
      ->method('getCurrentLanguage')
      ->will($this->onConsecutiveCalls($english, $english, $japanese, $japanese));
    $this->cache = new TestMemoryBackend();

    $this->assetResolver = new AssetResolver($this->libraryDiscovery, $this->libraryDependencyResolver, $this->moduleHandler, $this->themeManager, $this->languageManager, $this->cache);
  }

  /** * @covers ::getCssAssets * @dataProvider providerAttachedAssets */
  public function testGetCssAssets(AttachedAssetsInterface $assets_a, AttachedAssetsInterface $assets_b, $expected_cache_item_count) {
    $this->assetResolver->getCssAssets($assets_a, FALSE, $this->english);
    $this->assetResolver->getCssAssets($assets_b, FALSE, $this->english);