Make pub web socket API follow JSON-RPC 2.0

[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#17492

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Right now, it has an ad-hoc JSON structure. May as well follow a standard.

[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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Marked this as blocking dart-lang/sdk#15830.

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<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 #896.

[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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Set owner to @nex3.
Added Started label.

[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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dart-lang/sdk@889e7d3 creates a package that implements a JSON-RPC 2.0 server.

[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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dart-lang/sdk@54bb8fa converts pub's admin UI port's web socket API to use JSON-RPC 2.0.

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Added Fixed label.

[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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Unmarked this as blocking #896.