[WIP] Add ResourceRetriever::getFilePath()

[jslee02]

Motivation
In some cases, it is required to extract the (absolute) file path of a URI (if available). For example, MeshShape needs to know the path to a mesh file to infer the path to a texture image, which is given as a relative path. Currently, we don't have a public method to get a file path from a URI unless it's a file URI. PackageResourceRetriever and DartResourceRetriever have similar functionality, but it's not public.

Proposal
I propose adding ResourceRetriever::getFilePath() that returns the absolute file path specified by a URI or an empty string if unavailable.

---

Before creating a pull request

• Document new methods and classes

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[mxgrey]

We could consider adding a small unit test to ensure coverage of the new function, but it's likely already covered through the exists(~) function, so maybe it isn't important.

[jslee02]

Sure, I wanted to hear from you (and possibly @mkoval) if this sounds before adding tests. 😄

Btw, I meant to add this to DART 6.4 (and later by cascading the changes) but unintentionally made this branching from master. Let me create a new PR for DART 6.4. 😓