Combine/merge multiple maps into 1 map

you can use addAll method of Map object

var firstMap = {"1":"2"};
var secondMap = {"2":"3"};

var thirdMap = {};

thirdMap.addAll(firstMap);
thirdMap.addAll(secondMap);

print(thirdMap);

Or

var thirdMap = {}..addAll(firstMap)..addAll(secondMap);

Update

Since dart sdk 2.3 You can use spread operator ...

final firstMap = {"1":"2"};
final secondMap = {"2":"3"};

final thirdMap = {
   ...firstMap,
   ...secondMap,
};

alternate syntax using Map.addAll, Iterable.reduce and cascading operator, for combining a lot of maps:

var combinedMap = mapList.reduce( (map1, map2) => map1..addAll(map2) );

live dartpad example https://dartpad.dartlang.org/9cd116d07d2f45a9b890b4b1186dcc5e