How to stop Dart's .forEach()?

You can also use a for/in, which implicitly uses the iterator aptly demonstrated in the other answer:

List data = [1,2,3];

for(final i in data){
  print('$i');
  if (i == 2){
    break;
  }
}

It is also possible to implement your example using forEach() and takeWhile().

var data = [1, 2, 3];
data.takeWhile((val) => val != 2).forEach(print);

Breaking a List

List<int> example = [ 1, 2, 3 ];

for (int value in example) {
  if (value == 2) {
    break;
  }
}

Breaking a Map

If you're dealing with a Map you can't simply get an iterator from the given map, but you can still use a for by applying it to either the values or the keys. Since you sometimes might need the combination of both keys and values, here's an example:

Map<String, int> example = { 'A': 1, 'B': 2, 'C': 3 };

for (String key in example.keys) {
  if (example[key] == 2 && key == 'B') {
    break;
  }
}

Note that a Map doesn't necessarily have they keys as [ 'A', 'B', 'C' ] use a LinkedHashMap if you want that. If you just want the values, just do example.values instead of example.keys.

Alternatively if you're only searching for an element, you can simplify everything to:

List<int> example = [ 1, 2, 3 ];
int matched = example.firstMatching((e) => e == 2, orElse: () => null);