in src/Reflection/ReflectionXHPClass.hack [37:46]
public function getAttribute(string $name): ReflectionXHPAttribute {
$map = $this->getAttributes();
invariant(
C\contains_key($map, $name),
'Tried to get attribute %s for XHP element %s, which does not exist',
$name,
$this->getClassName(),
);
return $map[$name];
}