Barback asset conflict message could be more clear

[blois]

I've gotten stumped by failing builds twice because I had existing dart2js output in the folder when running pub build, would be great if the error message would be more clear.

Currently it just says "Transforms $transforms all emitted asset $id."

Would be great if this could say something about it conflicting with an existing file.

I'd be happy to update the string if someone has a decent suggestion for what it should be.

[nex3]

Added Duplicate label.
Marked as being merged into #14593.

[anders-sandholm]

Removed Library-Barback label.
Added Pkg-Barback label.

[DartBot]

This issue has been moved to dart-archive/barback#30.