Add pre and post install hooks for Pub

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="96" height="96"hspace="10"> Issue by sethladd
Originally opened as dart-lang/sdk#4674

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Request from the mailing list:

Pub manager feature request.
Add 'preinstall' and 'uninstall' sections into pubspec.yaml.
This can be useful when installing the packages with native extensions.
Example.
--pubspec.yaml
name: package_with_native_ext
preinstall: my_preinstall_script
-- eof

--my_preinstall_script.dart
main() {
  switch(Platform.operatingSystem) {
    case 'linux':
       copyBinaryFilesToPackageDirForCorrectWorkInLinux();
       break;
    // etc
  }
}
-- eof

[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 this to the Later milestone.

[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/2311549?v=3" align="left" width="48" height="48"hspace="10"> Comment by mezoni

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Somehow, this is not moving (70 days).
It's elementary.
Really absolutely no options to start with something?

[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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Merging this with 3023 because at least in the description the use case here seems to be building native extensions.

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Added Duplicate label.
Marked as being merged into #39.