Provide a way of getting packageRoot as a Uri

[johnniwinther]

Platform.packageRoot should return a Uri.

Currently is is passed on verbatim from the system, and even though the package root always refers to a directory-like resource every user must ensure that it is normalized before handling.

For instance, you cannot use

Uri packageRoot = Uri.base.resolve(Platform.packageRoot);

because it might be missing a trailing slash and might contain ''.

[kevmoo]

We can't change the type returned from packageRoot – we want to avoid breaking changes.

We could add a packageRootUri property or similar, though

---

Added Library-IO label.

[andersjohnsen]

Consider using 'new Uri.file(Platform.packageRoot)' to get the Uri you need. You will still need to look for tailing slash though.

[johnniwinther]

For a Platform-independent version without double slashes that would require:

Uri fileUri = Uri.base.resolveUri(
    new Uri.file(Platform.packageRoot, windows: Platform.isWindows));
Uri dirUri = fileUri.path.endWith('/') ? fileUri
    : new Uri(scheme: fileUri.scheme, path: '${fileUri.path}/');

This is, and will be, handled incorrectly in many use-cases.

[andersjohnsen]

  Uri fileUri = Uri.base.resolveUri(new Uri.file(Platform.packageRoot));
  Uri dirUri = fileUri.path.endWith('/') ? fileUri
      : new Uri(scheme: fileUri.scheme, path: '${fileUri.path}/');

Should be sufficient.

But I agree, it's non-trivial.

[sethladd]

cc @iposva-google

[sethladd]

For those following along, this getter will start to return a URI if the VM has resolved packageRoot, even if --package-root was not set on the command line. In other words, if the VM has decided to use a package root, then packageRoot will return something.