This method is used internally when IDs are allocated to existing instances
of a Key. It should not generally be used externally.
Example:
$key = $datastore->key('Person');
$key->setLastElementIdentifier('Bob', Key::TYPE_NAME);
public function testSetLastElementIdentifierTypeName()
{
$key = new Key('foo', ['path' => [['kind' => 'foo', 'id' => 2], ['kind' => 'foo']]]);
$key->setLastElementIdentifier(1, Key::TYPE_NAME);
$this->assertEquals($key->path(), [['kind' => 'foo', 'id' => 2], ['kind' => 'foo', 'name' => 1]]);
}