extend Resource API to return the resolved uri

[devoncarew]

Right now the Resource uri returns the uri that was passing into the constructor (https://api.dartlang.org/1.12.0/dart-core/Resource-class.html). It would be nice if it could also return the uri that the resource resolves to. I.e. the ctor takes a package: uri, which likely resolves to a file: or an http: uri.

This information can be useful for tools that do need to know a bit more context about where the resources actually live. An example is an osgi / bundle loader for dart. It might want to try and locate the pubspec.yaml files for the installed packages. Right now only code and resources in lib/ are available to tools.

[devoncarew]

cc @sethladd

[sethladd]

Thanks! Dupe of #23990