title: How to set a custom property for conditional import in Dart?
body: <p>Dart allows to use the standard library names for conditional import/export, like this:</p>
  <pre><code>export 'src/hw_none.dart' // Stub implementation
      if (dart.library.io) 'src/hw_io.dart' // dart:io implementation
      if (dart.library.html) 'src/hw_html.dart'; // dart:html implementation
  </code></pre>
  <p>Is it possible to define a custom property/condition? For example, pass it when compiling.</p>
  <p>I have a project that I would like to split into two variants: Flutter variant and pure Dart variant. The choice of the variant depends at compile time, and the necessary implements of abstract classes  defines at compile time.</p>