Add an as() for null-safety casting similar to as? in Kotlin

[yongjhih]

T as<T>(dynamic it) => it is T ? it : null; // that's what I used for a long time.

  test('should as<T>()', () async {
    const dynamic it = "Hello";
    expect(as<String>(it) ?? "", "Hello");
    expect(as<int>(it) ?? 0, 0);
    const nullIt = null;
    expect(as<String>(nullIt) ?? "", "");
  });

I tried to implement it with the following code, but they don't work, I think it's a limitation in Dart:

extension NullableObject on Object {
  R as<R>() => this is R ? this as R : null;
}

or

extension NullableDynamic on dynamic {
  R as<R>() => this is R ? this as R : null;
}

[yongjhih]

dart-lang/language#399 (comment)