Class 'yii\base\UnknownClassException' not found

While using `AspectMock` for unit testing, the `autoload` method in `BaseYii.php` can not find the following class:
> Class 'yii\base\UnknownClassException' not found

However tests are passing successfully.
```
public static function autoload($className)
{
if (isset(static::$classMap[$className])) {
$classFile = static::$classMap[$className];
if ($classFile[0] === '@') {
$classFile = static::getAlias($classFile);
}
} elseif (strpos($className, '\\') !== false) {
$classFile = static::getAlias('@' . str_replace('\\', '/', $className) . '.php', false);
if ($classFile === false || !is_file($classFile)) {
return;
}
} else {
return;
}

include $classFile;

if (YII_DEBUG && !class_exists($className, false) && !interface_exists($className, false) && !trait_exists($className, false)) {
throw new UnknownClassException("Unable to find '$className' in file: $classFile. Namespace missing?"); // <---
}
}
```
[Test snapshot](https://www.screencast.com/t/1suVpY4O1bj)

Start with the autoload method in BaseYii.php and reproduce the reported AspectMock unit-testing case shown in the linked test snapshot. Trace why UnknownClassException cannot be found even though the tests pass; done means the reported autoload failure is resolved without regressing the existing passing tests.

Written by the indexing model from the issue text.