Dart - About executing multiple lines of processing in an if statement

I am trying with https://dartpad.dev/.

I want to execute multiple processes in the if statement as follows.

void main() {
  List<dynamic> arrayA = [1, 2, 3];
  List<dynamic> arrayB = [1, 2, 5, 6];
  arrayB.forEach((m) => {
        if (!arrayA.contains(m)) {
          print(m);
          break;
        }
      });
}

However, the following error is output when the process of multiple lines is written in the if statement.

Error compiling to JavaScript:
main.dart:6:19:
Error: Expected '}' before this.
          print(m);
                  ^
Error: Compilation failed.

What I want to do is following..

(Temporarily, the process you want to execute is "A" (in this case, "print(m);"))

-In the loop, execute the processing "A" when the condition is true

-I want to escape from the loop after executing the first "A".

Then, please tell me what is wrong in the code and how to write it correctly.