Issue title: [BUG] load license error
 Issue body: **Describe the bug**
I recieve this message error "Flutter Error (Unable to load asset: google_fonts/OFL.txt)" everytime the project try to execute the load.

LicenseRegistry.addLicense(() async* {
    final license = await rootBundle.loadString('google_fonts/OFL.txt');
    yield LicenseEntryWithLineBreaks(['google_fonts'], license);
  });

Is it need any other configuration?
 Comments: 
 Comment 0: Hi, that file needs to exist, and you should add it to your [assets](https://api.flutter.dev/flutter/services/rootBundle.html). Please re-open if it still doesn't work!
{'source': 'GithubIssues', 'repo_name_with_owner': 'material-foundation/google-fonts-flutter', 'issue_id': 1048679857, 'repo_stars': 664.0}