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Flow: spreads don't preserve read-only-ness

type A = {| +readOnlyKey: string |};
type B = {| ...A, +otherKey: string |};

function test(x: B) {
x.readOnlyKey = 'overwrite'; // no error ?
x.otherKey = 'overwrite'; // no error ??

This applies to value spreads as well since they are creating a new object. It's less understandable for these type spreads where value spread is not involved.