Names of anonymous libraries in 'Flutter' package are not stripped of 'package' prefix

[srawlins]

As @goderbauer found earlier, when removing the name portion of a library directive, e.g. s/library widgets;/library; changes the URI of the generated docs for the widgets library.

It turns out this should have worked; there is code in Library.dirName that picks a "dir name" for an anonymous library. That code should strip the package prefix from libraries in the "default package." When generating docs for the Flutter SDK, the Flutter "package" is considered "default," but it's name is capitalized: "Flutter." Because of this, the stripping behavior does not work.

This issue prevents the flutter SDK from adopting the unnecessary_library_names lint rule.