pub serve should show build errors in the browser

[DartBot]

<img src="https://avatars.githubusercontent.com/u/522948?v=3" align="left" width="96" height="96"hspace="10"> Issue by justinfagnani
Originally opened as dart-lang/sdk#19490

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If you try to load an asset from pub serve that fails to build because of an error you see a fairly unhelpful 404 message. It would be much better if pub could remember which errors occurred for which assets and show them in its response.

When used form the Editor, the build error message can be lost in the quite verbose tools output, leading one to try to figure out why the Editor is getting a 404 for a file that's clearly present - you're editing it after all. This is a pretty bad overall user experience.

[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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Marked this as being blocked by dart-archive/barback#54.

[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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Removed Type-Defect label.
Added Type-Enhancement label.

[matanlurey]

We are no longer supporting transformers or pub serve/build going forward (this tooling has moved elsewhere into the https://github.com/dart-lang/build repository and https://github.com/dart-lang/webdev CLI) so a fix for this is not planned.

We'll be updating the tooling section of the Dart website in the coming weeks with more formal documentation on how to get started.