pub serve should detect changes to pubspec.yaml

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#19214

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Changing the pubspec should practically trigger a restart of barback and recompute anything that's needed.

See issue dart-archive/polymer-dart#204 for an example: a user creates a file, creates a transform section in the pubspec, later decides to rename the file and update the corresponding section in the pubspec.

[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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Added Duplicate label.
Marked as being merged into dart-lang/sdk#19163.