diff --git a/core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php b/core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php
new file mode 100644
index 0000000..00dbdf5
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php
@@ -0,0 +1,173 @@
+<?php
+
+namespace Drupal\Tests\Core\Assert;
+
+use Behat\Mink\Element\DocumentElement;
+use Behat\Mink\Element\NodeElement;
+use Behat\Mink\Session;
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\FunctionalTests\AssertLegacyTrait;
+use Drupal\Tests\UnitTestCase;
+use Drupal\Tests\WebAssert;
+use PHPUnit_Framework_ExpectationFailedException;
+
+/**
+ * @coversDefaultClass \Drupal\FunctionalTests\AssertLegacyTrait
+ * @group Assert
+ */
+class AssertLegacyTraitTest extends UnitTestCase {
+
+  use AssertLegacyTrait;
+
+  /**
+   * The mocked Mink session object used for testing.
+   *
+   * @var \Behat\Mink\Session|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $session;
+
+  /**
+   * The mocked page element used for testing.
+   *
+   * @var Behat\Mink\Element\DocumentElement|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $page;
+
+  /**
+   * The mocked web assert class.
+   *
+   * @var \Drupal\Tests\WebAssert|\Prophecy\Prophecy\ObjectProphecy
+   */
+  protected $webAssert;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    $this->page = $this->prophesize(DocumentElement::class);
+    $this->session = $this->prophesize(Session::class);
+    $this->session->getPage()->willReturn($this->page->reveal());
+    $this->webAssert = $this->prophesize(WebAssert::class);
+  }
+
+  /**
+   * @covers ::assertUniqueText
+   */
+  public function testAssertUniqueText() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $this->assertUniqueText('foo');
+  }
+
+  /**
+   * @covers ::assertUniqueText
+   */
+  public function testAssertUniqueTextFail() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $this->setExpectedException(PHPUnit_Framework_ExpectationFailedException::class);
+    $this->assertUniqueText('bar');
+  }
+
+  /**
+   * @covers ::assertUniqueText
+   */
+  public function testAssertUniqueTextUnknown() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $this->setExpectedException(PHPUnit_Framework_ExpectationFailedException::class);
+    $this->assertUniqueText('alice');
+  }
+
+  /**
+   * @covers ::assertUniqueText
+   */
+  public function testAssertUniqueTextMarkup() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $markupObject = $this->prophesize(MarkupInterface::class);
+    $markupObject->__toString()->willReturn('foo');
+    $this->assertUniqueText($markupObject->reveal());
+  }
+
+  /**
+   * @covers ::assertNoUniqueText
+   */
+  public function testAssertNoUniqueText() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $this->assertNoUniqueText('bar');
+  }
+
+  /**
+   * @covers ::assertNoUniqueText
+   */
+  public function testAssertNoUniqueTextFail() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $this->setExpectedException(PHPUnit_Framework_ExpectationFailedException::class);
+    $this->assertNoUniqueText('foo');
+  }
+
+  /**
+   * @covers ::assertNoUniqueText
+   */
+  public function testAssertNoUniqueTextUnknown() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $this->setExpectedException(PHPUnit_Framework_ExpectationFailedException::class);
+    $this->assertNoUniqueText('alice');
+  }
+
+  /**
+   * @covers ::assertNoUniqueText
+   */
+  public function testAssertNoUniqueTextMarkup() {
+    $this->page->getText()->willReturn('foo bar bar');
+    $markupObject = $this->prophesize(MarkupInterface::class);
+    $markupObject->__toString()->willReturn('bar');
+    $this->assertNoUniqueText($markupObject->reveal());
+  }
+
+  /**
+   * @covers ::assertOptionSelected
+   */
+  public function testAssertOptionSelected() {
+    $option_field = $this->prophesize(NodeElement::class);
+    $option_field->hasAttribute('selected')->willReturn(TRUE);
+
+    $this->webAssert
+      ->optionExists('myselect', 'two')
+      ->willReturn($option_field->reveal());
+
+    $this->assertOptionSelected('myselect', 'two');
+  }
+
+  /**
+   * @covers ::assertOptionSelected
+   */
+  public function testAssertOptionSelectedFail() {
+    $option_field = $this->prophesize(NodeElement::class);
+    $option_field->hasAttribute('selected')->willReturn(FALSE);
+
+    $this->webAssert
+      ->optionExists('myselect', 'two')
+      ->willReturn($option_field->reveal());
+
+    $this->setExpectedException(PHPUnit_Framework_ExpectationFailedException::class);
+    $this->assertOptionSelected('myselect', 'two');
+  }
+
+  /**
+   * Returns a mocked behat session object.
+   *
+   * @return \Behat\Mink\Session
+   *   The mocked session.
+   */
+  protected function getSession() {
+    return $this->session->reveal();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function assertSession($name = NULL) {
+    return $this->webAssert->reveal();
+  }
+
+}