Support installing a new package directly from command line

[DartBot]

Originally opened as dart-lang/sdk#8815

This issue was originally filed by timarms...@google.com

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It would be nice if we could do something like

pub install web_ui

and it would add the dependency to the bottom on the local pubspec.yaml (or create a pubspec if one doesn't exist), and then it would perform the normal installation. Extra params with that command could be used to specify versions, etc.

[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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Right now, pub doesn't ever actually modify a pubspec. Programmatically modifying a file that a user hand-edits as well tends to be pretty nasty, especially with a flexible language like YAML. I think it's unlikely that we'll want to do that in pub.

It would be useful to be able to tell pub to install a package to the system cache, though, without touching the pubspec. That could be useful if you know you're about to not have network access and want to "pre-cache" a package.

Once we have better support for packages that expose command-line apps, it will also be necessary to be able to install and use a package that you don't have an actual dependency on.

I'll leave this bug open for that latter case.

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Added this to the Later milestone.
Removed Priority-Low label.
Added Priority-Medium label.

[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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Added Community-StarterBug label.

[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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Added Duplicate label.
Marked as being merged into #407.