src/DI/Storage/TypeDefinition.php
public function isSingleton($type): bool
    {
        assert(isset($this->types[$type]));
        return $this->types[$type];
    }
public function isSingleton($type): bool
    {
        assert(isset($this->types[$type]));
        return $this->types[$type];
    }
public function isSingleton ( $ type ) : bool { assert ( isset ( $ this -> types [ $ type ] ) ) ; return $ this -> types [ $ type ] ; }
Only `DI\Provider` can call this function.
So, `DI\Provider` is responsible to the assertion.

@param $type
@return bool
Only DI \ Provider can call this function . So DI \ Provider is responsible to the assertion .