Call to undefined method Drupal\Core\Config\Schema\Undefined::get

We ran into the following error when updating from 8.2.x to 8.3.0-rc2

Error: Call to undefined method Drupal\Core\Config\Schema\Undefined::get() in web/core/lib/Drupal/Core/Config/StorableConfigBase.php on line 179 #0
web/core/lib/Drupal/Core/Config/Config.php(212):
Drupal\Core\Config\StorableConfigBase->castValue('bc_primitives_a...', false)

Debuging the code revealed this in class TypedConfigManager

public function hasConfigSchema($name) {
    // The schema system falls back on the Undefined class for unknown types.
    $definition = $this->getDefinition($name);
    return is_array($definition) && ($definition['class'] != Undefined::class);
  }

In some cases $definition['class'] was coming out as undefined, which should compare to Undefined::class,
but when debugging the actual values for each we see:

'\Drupal\Core\Config\Schema\Undefined' and 'Drupal\Core\Config\Schema\Undefined'

Further debug showed that the Undefined class definition is not being run through the processDefinition method and is therefore not being stripped of the leading \.

We should maybe do something like:

(ltrim($definition['class'], '\\') !== Undefined::class)

We have cleared the fault by clearing caches before doing the upgrade, however felt we should document this as a bug.