foo.dart

// Dynamically injects an html import, but gives the warning
// `URL must be imported by main Dart script: foo.dart`
@HtmlImport('foo.html')
library foo;

import 'package:web_components/web_components.dart';

foo.html

<script type="application/dart" src="foo.dart"></script>

index.dart

import 'package:foo/foo.dart';
import 'package:initialize/initialize.dart' as init;

main() {
  // Crawls imports looking for @HtmlImport and does dynamic injection
  init.run();
}

index.html

<script src="application/dart" href="index.dart"></script>