pkg/yaml: format-preserving editing of parsed yaml

[DartBot]

<img src="https://avatars.githubusercontent.com/u/444270?v=3" align="left" width="96" height="96"hspace="10"> Issue by seaneagan
Originally opened as dart-lang/sdk#20998

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This would be useful for issue dart-lang/pub#982 and issue dart-lang/pub#1135.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/8631949?v=3" align="left" width="48" height="48"hspace="10"> Comment by floitschG

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Added Area-Library, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/4865287?v=3" align="left" width="48" height="48"hspace="10"> Comment by lrhn

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Removed Area-Library label.
Added Area-Pkg, Pkg-Yaml labels.

[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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This would be great, but I doubt I'll have time to work on it anytime soon.

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Removed Priority-Unassigned label.
Added Priority-Low label.