Support running pub serve on directories other than "web/"

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<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#15924

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This is the pub serve dual of #­14673.

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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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It's possible this should work somewhat differently for "pub serve" than for "pub build". For building the primary use-case for building a specific directory is to take the output -- for example, an example app -- and put it on a server somewhere. This means that saying e.g. "pub build example/foo" makes a lot of sense. For "pub serve", though, it may be more useful to just have the server always make the examples and tests accessible. You'd still run "pub serve" without any arguments, but then you could go to "http://localhost/_pkg/example/foo" (or something, "_pkg" is just a strawman).

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<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#15859.

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<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 dart-lang/sdk#15859.

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<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/46275?v=3" align="left" width="48" height="48"hspace="10"> Comment by munificent

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Added DevStory label.

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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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Set owner to @nex3.
Added Started label.

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<img src="https://avatars.githubusercontent.com/u/2192312?v=3" align="left" width="48" height="48"hspace="10"> Comment by keertip

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Marked this as blocking dart-lang/sdk#16889.

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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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Support for this has been added in dart-lang/sdk@2d1a8c7. "pub serve" can now serve files from multiple directories, each on a different port. By default it serves "web" on port 8080 and "test" on port 8081; additional command-line arguments can be passed to cause it to serve other directories. For example, "pub serve example/foo" will serve example/foo on port 8082 (8080 and 8081 are always reserved for "web" and "test").

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Added Fixed label.