guid function
return a random guid.
Implementation
String guid() {
const self = 'abcdefABCDEF1234567890';
return '${string(pools: [], self: self, min: 8)}-${string(pools: [], self: self, min: 4)}-${string(pools: [], self: self, min: 4)}-${string(pools: [], self: self, min: 12)}';
}