Tzip12Contract extends Tzip16Contract . It provides all the methods in the Tzip16Contract and Contract Class. Along with that is provides getTokenMetadata.
You can get Token meta data by calling getTokenMetadata on the Tzip12Contract Instance.
var tzip12Contract = Tzip12Contract(
address: 'KT1RJ6PbjHpwc3M5rw5s2Nbmefwbuwbdxton',
rpcServer: '<https://mainnet.api.tez.ie>');
print(await tzip12Contract.getTokenMetadata(1392));