in scrooge-generator-typescript/src/main/scala/com/gu/scrooge/backend/typescript/TypescriptGenerator.scala [215:221]
def needsInt64Import(ft: Seq[FieldType]): Boolean = ft.exists {
case TI64 => true
case listType: ListType => needsInt64Import(Seq(listType.eltType))
case setType: SetType => needsInt64Import(Seq(setType.eltType))
case mapType: MapType => needsInt64Import(Seq(mapType.keyType, mapType.valueType))
case _ => false
}