Do not require FLUTTER_ROOT any longer, even for flutter packages

[srawlins]

I saw this TODO from @devoncarew, and tried it myself: I can run dart doc, with a dart script from the flutter/flutter repo, and no FLUTTER_ROOT env variable is necessary. Swoosh!

Also delete the test for this exception. And then delete the testing/ package, which was only used by that last test.

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[kallentu]

LGTM.

Could we remove the flutterRoot option? Is that still needed?

[srawlins]

Could we remove the flutterRoot option? Is that still needed?

I think the flutterRoot option (which is typically set via FLUTTER_ROOT, in dartdoc_options.dart, can still serve a purpose if you are manually overriding the default value. I don't know if there is a reason to do this. We could deprecate it (since FLUTTER_ROOT is no longer required), and see if it causes any problems.