meta 1.3.0-nullsafety.2

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Introduce @sealed to declare that a class or mixin is not allowed as a super-type.

Only classes in the same package as a class or mixin annotated with @sealed may extend, implement or mix-in the annotated class or mixin. (SDK issue 27372).

Introduce @alwaysThrows to declare that a function always throws (SDK issue 17999). This is first available in Dart SDK 1.25.0-dev.1.0.

import 'package:meta/meta.dart';

// Without knowing that [failBigTime] always throws, it looks like this
// function might return without returning a bool.
bool fn(expected, actual) {
  if (expected != actual)
    failBigTime(expected, actual);
  else
    return True;
}

@alwaysThrows
void failBigTime(expected, actual) {
  throw new StateError('Expected $expected, but was $actual.');
}

Introduce @virtual to allow field overrides in strong mode (SDK issue 27384).

import 'package:meta/meta.dart' show virtual;
class Base {
  @virtual int x;
}
class Derived extends Base {
  int x;

  // Expose the hidden storage slot:
  int get superX => super.x;
  set superX(int v) { super.x = v; }
}

Introduce @checked to override a method and tighten a parameter type (SDK issue 25578).

import 'package:meta/meta.dart' show checked;
class View {
  addChild(View v) {}
}
class MyView extends View {
  // this override is legal, it will check at runtime if we actually
  // got a MyView.
  addChild(@checked MyView v) {}
}
main() {
  dynamic mv = new MyView();
  mv.addChild(new View()); // runtime error
}