Avoid annotating with dynamic when not requiredDRT-W1045

AVOID annotating with dynamic when not required.

As dynamic is the assumed return value of a function or method, it is usually not necessary to annotate it.

BAD:

dynamic lookUpOrDefault(String name, Map map, dynamic defaultValue) {
  var value = map[name];
  if (value != null) return value;
  return defaultValue;
}

GOOD:

lookUpOrDefault(String name, Map map, defaultValue) {
  var value = map[name];
  if (value != null) return value;
  return defaultValue;
}