Backport forward compatibility layer for PHPUnit 6 from PHPUnit 5

src/ForwardCompatibility/Assert.php

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+<?php
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace PHPUnit\Framework;
+
+use PHPUnit_Framework_Assert;
+
+abstract class Assert extends PHPUnit_Framework_Assert
+{
+}

src/ForwardCompatibility/BaseTestListener.php

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+<?php
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace PHPUnit\Framework;
+
+use PHPUnit_Framework_BaseTestListener;
+
+abstract class BaseTestListener extends PHPUnit_Framework_BaseTestListener
+{
+}

src/ForwardCompatibility/TestCase.php

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+<?php
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace PHPUnit\Framework;
+
+use PHPUnit_Framework_TestCase;
+
+abstract class TestCase extends PHPUnit_Framework_TestCase
+{
+}

src/ForwardCompatibility/TestListener.php

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+<?php
+/*
+ * This file is part of PHPUnit.
+ *
+ * (c) Sebastian Bergmann <sebastian@phpunit.de>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace PHPUnit\Framework;
+
+use PHPUnit_Framework_TestListener;
+
+interface TestListener extends PHPUnit_Framework_TestListener
+{
+}