Site-www: Update Effective Dart for null safety

[kwalrath]

Examples:

• Don't use late final for a local variable.
• Use : syntax in constructors when possible, rather than late final.

What other examples might we want?

/cc @munificent @mit-mit @johnpryan @kevmoo

[kevmoo]

It's more like avoid late final if it's not needed. It may be a subtle thing to communicate.

[munificent]

Another example from @srawlins:

String? s = "hello";
if (s?.isNotEmpty ?? false) {
  // s is not promoted to String.
  print(s!.length);
}

means that variables are not promoted to non-nullable. But if you re-wrote it:

String? s = "hello";
if (s != null && s.isNotEmpty) {
  // s is promoted to String.
  print(s.length);
}

then you get nice promotion. This only works for variables, not for property access, so list != null && list.isNotEmpty && list.first != null does not promote list.first. But I think non-nullable promotions are a very good reason to change our thinking on ?? false.

[kwalrath]

Marking this p2 because Effective Dart often lags a bit... but I'd love to have this ready for the stable release.