title: Dart - assign value only if null (??=) gives warning message
body: <p>When I execute this small dart program:</p>
  <pre><code>void main() {
    int? someint;
    someint ??= 123;
    someint ??= 246;
    print(someint);
  }
  </code></pre>
  <p>I get the following warning message;</p>
  <pre><code>$ dart test.dart 
  test.dart:4:3: Warning: Operand of null-aware operation '??=' has type 'int' which excludes null.
    someint ??= 246;
    ^
  123
  </code></pre>
  <p>So everything works as expected, but the warning message is annoying. Is there any way to suppress this warning message?</p>