Add pre and post install hooks for Pub

[sethladd]

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

[nex3]

Added this to the Later milestone.

[munificent]

Removed Type-Defect label.
Added Type-Enhancement label.

[DartBot]

This comment was originally written by @mezoni

---

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

[munificent]

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

---

Added Duplicate label.
Marked as being merged into #3023.

[DartBot]

This issue has been moved to dart-lang/pub#69.

[sethladd]

For those following along, work on a feature that would support the spirit behind this request can be found here:

dart: #50565
flutter: flutter/flutter#129757