So there are multiple issues hiding here:
They don't actually really run under drt, it is just that the wrapping messed this up. Martin just landed a fix for this.
The real problem is that when we call spawnUri, the files are loaded relative to either the main dart script or relative to the html file (in our case, that does not really matter, since both of these are not located next to the file with the test)

I don't really see how spawnUri can be really useful in packages like this, they have no way of knowing what to relative load something from (as it is now, it will be wrong unless you hardcode some path in and assume that users restrict the location of the package relative to the main script).

Florian: Since you are changing this stuff, please let us know what you come up with.

cc @mkustermann.
cc @munificent.
Removed Priority-Unassigned, Area-Test labels.
Added Priority-Medium, Library-Async labels.

Added Area-Library label.

Libraries cannot really use spawnUri with a relative URL because they have no control over the base URL used, and they don't have an easy way to access their own URL (you can get it using mirrors).

We should consider making a way for a library to find its own URL without going through mirrors. That will allow it to load files that are part of the same library using relative URLs.

Added Duplicate label.
Marked as being merged into #15974.