prefer_const_constructors  
            
            Prefer const with constant constructors.
This rule is available as of Dart 2.0.0.
Rule sets: flutter
This rule has a quick fix available.
Details
PREFER using const for instantiating constant constructors.
If a constructor can be invoked as const to produce a canonicalized instance, it’s preferable to do so.
BAD:
class A {
  const A();
}
void accessA() {
  A a = new A();
}
GOOD:
class A {
  const A();
}
void accessA() {
  A a = const A();
}
GOOD:
class A {
  final int x;
  const A(this.x);
}
A foo(int x) => new A(x);
Usage
To enable the prefer_const_constructors rule,
add prefer_const_constructors under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - prefer_const_constructors