Web admin interface for pub serve

[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#16954

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We've wanted to have a web interface as a window into pub serve for a long time. Making it full featured will be many separate bugs. This one is just for the minimal first steps:

1. When pub serve is run, bind a web server to :8000.
2. That initially just shows a list of the root directories being served with links to their appropriate URLs.

[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 blocking #873.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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cc @danrubel.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Unmarked this as blocking #873.

[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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Issue #937 has been merged into this issue.

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cc @munificent.

[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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Renaming so searching for "admin" finds this issue.

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Changed the title to: "Web admin interface for pub serve".

[DartBot]

<img src="https://avatars.githubusercontent.com/u/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Issue #930 has been merged into this issue.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/648527?v=3" align="left" width="48" height="48"hspace="10"> Comment by Fox32

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One use case for the admin interface, or more specific the WebSocket server:

I would like to subscribe for changes to assets and get notified, so that I can request them when they have changed. That way I could see changes intermediately.

[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.