title: Flutter compile error: non-null value must be returned since the return type 'String' doesn't allow null - displayString
body: <p>My tests are failing to compile and run in a Dart only project that is referenced by my Flutter project.  I'm receiving the following error message</p>
  <pre><code>Failed to precompile test:test:
  ../../../../../../../../../.pub-cache/hosted/pub.dartlang.org/analyzer-1.0.0/lib/src/error/best_practices_verifier.dart:1952:14: Error: A non-null value must be returned since the return type 'String' doesn't allow null.
    String get displayString {
  </code></pre>
  <p>Any ideas please?</p>
  <pre><code>Flutter 2.2.1 (current stable channel)
  Tools • Dart 2.13.1
  </code></pre>
  <p>(I've asked the question in Flutter's github here also)
  <a href="https://github.com/flutter/flutter/issues/83683" rel="nofollow noreferrer">https://github.com/flutter/flutter/issues/83683</a></p>