avoid_annotating_with_dynamic   
            
            Avoid annotating with dynamic when not required.
This rule is available as of Dart 2.0.0.
This rule has a quick fix available.
Details
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;
}
Usage
To enable the avoid_annotating_with_dynamic rule,
add avoid_annotating_with_dynamic under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - avoid_annotating_with_dynamic