Barback asset conflict message could be more clear

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2152569?v=3" align="left" width="96" height="96"hspace="10"> Issue by blois
Originally opened as dart-lang/sdk#16374

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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.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="48" height="48"hspace="10"> Comment by nex3

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Added Duplicate label.
Marked as being merged into #18.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/3276024?v=3" align="left" width="48" height="48"hspace="10"> Comment by anders-sandholm

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