Need tests for code that determines what files are published with a package

[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#7391

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The code for getting the list of files to include in a package has a bunch of interesting corners related to symlinks, git, etc. We've seen a few issues there where it doesn't give us the files we expect, and we don't have good test coverage for it. We should. In particular:

1. Make sure packages directories are excluded.
2. Make sure symlinks are handled correctly.
3. Make sure .gitignored files are ignored.
4. Make sure .gitignored directories (and their contents) are ignored.
5. Make sure "." files are ignored in non-git packages.

[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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1. Make sure "pubspec.lock" is always omitted.

[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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Removed Type-Defect label.
Added Type-Enhancement 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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Removed this from the M3 milestone.
Added this to the M4 milestone.

[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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We should test this as part of our "pub lish" integration tests, but right now we have no way of parsing a multipart/form-data request.

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Marked this as being blocked by dart-lang/sdk#2488.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/1343914?v=3" align="left" width="48" height="48"hspace="10"> Comment by sgjesse

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Marked this as being blocked by dart-lang/sdk#8834.
Unmarked this as being blocked by dart-lang/sdk#2488.

[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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Removed this from the M4 milestone.
Added this to the M5 milestone.

[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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Fixed by dart-lang/sdk@f878dcc.

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Added Fixed label.