`@pragma` for dart analyzer to hint null type promotion.

Hi,

the following code will work perfectly fine with current Dart 3.1.3:

String? foo = null; 

if (foo != null) {
  String x = foo; // works, no ! needed
}

Now, I often want to check if the String has content, which leads to:

String? foo = null; 

if (foo != null && foo.isNotEmpty) {
  String x = foo; // works, no ! needed
}

Since I use that quite often in my app, I thought it might be nice to have an extension method for that:

extension NullableStringExtension on String? {
  bool get hasContent => this != null && this!.isNotEmpty;
}

The usage should be:

String? foo = null;

if (foo.hasContent) {
  String x = foo; // Urgh, does not work, because Dart thinks, that foo could be null.
}

However, this does not work, because the analyzer still thinks, that the foo could be null. Since hasContent already checks for that and only returns true, when foo is not null, it would be nice, if either the analyzer can actually analyze it, or it would be nice, if there was some pragma, like:

extension NullableStringExtension on String? {
  @pragma('not-null-when-true')
  bool get hasContent => this != null && this!.isNotEmpty;
}

This is similar to a C# feature: NotNullWhen.

It would be great so have something similar in Dart as well.

Start with the Dart 3.1.3 null-safety examples in the issue and review the proposed @pragma('not-null-when-true') behavior alongside the linked C# NotNullWhen reference. Done would require a language and analyzer design that lets foo be treated as non-null after hasContent evaluates true.