for_in_of_invalid_type

The type '{0}' used in the 'for' loop must implement '{1}'.

The analyzer produces this diagnostic when the expression following in in a for-in loop has a type that isn't a subclass of Iterable.

The following code produces this diagnostic because m is a Map, and Map isn't a subclass of Iterable:

void f(Map<String, String> m) {
  for (String s in m) {
    print(s);
  }
}

Replace the expression with one that produces an iterable value:

void f(Map<String, String> m) {
  for (String s in m.values) {
    print(s);
  }
}