Although Effective Dart recommends type annotations for public APIs, the function still works if you omit the types:

isNoble(atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}
For functions that contain just one expression, you can use a shorthand syntax:

bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;
The => expr syntax is a shorthand for { return expr; }. The => notation is sometimes referred to as arrow syntax.