spawnUri() canonicalization makes it difficult to use in libraries

[justinfagnani]

spawnUri() canonicalizes URIs to that they are relative to the application. This makes it difficult to spawnURI relative to the library containing the call without somehow knowing the relative location of the library to the application.

[dgrove]

Added Library-Isolate label.

[dgrove]

Removed Library-Isolates label.

[lrhn]

We still need a way for a library to know its own URI, or a way to create URIs relative to itself.

[lrhn]

Added Duplicate label.
Marked as being merged into #8440.