An asset whose path is a prefix of another should cause a collision

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="96" height="96"hspace="10"> Issue by munificent
Originally opened as dart-lang/sdk#17150

---

Since barback doesn't have a concept of directories, it's possible to emit two assets like:

foo/bar // a file "bar" with no extension
foo/bar/baz.txt

That will fail horribly with something like pub build when it tries to output those two files. Instead of allowing this, barback should probably treat that like a collision.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

---

Added Area-Pkg label.