Make it easy to run a custom pub server within an organization

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="96" height="96"hspace="10"> Issue by kevmoo Originally opened as dart-lang/sdk#22285

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Should be able to proxy to the production pub site when packages are not local

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Added C11 label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Work has started here https://github.com/dart-lang/pub_server/

We are discussing plans for a simple file-based solution. See this issue: https://github.com/dart-lang/pub_server/issues/2