pub run should save snapshots of dependencies' executables

[DartBot]

<img src="https://avatars.githubusercontent.com/u/188?v=3" align="left" width="96" height="96"hspace="10"> Issue by nex3
Originally opened as dart-lang/sdk#20482

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As long as a dependency doesn't transitively depend on the root package or a path dependency, that dependency's executables should be precompiled on "pub get"/"pub upgrade" and cached as VM snapshots.

Since snapshots aren't compatible across VM versions. If possible, we should detect version incompatibility by trying and failing to run a newer VM on an older snapshot.

[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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dart-lang/sdk@9f554af mostly implements this, but there are a few details I want to improve before closing out the bug.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/405837?v=3" align="left" width="48" height="48"hspace="10"> Comment by zoechi

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Please don't forget to add a note that .pub/ should be added to .gitignore

[DartBot]

<img src="https://avatars.githubusercontent.com/u/405837?v=3" align="left" width="48" height="48"hspace="10"> Comment by zoechi

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Performance is fine now !

[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@9f554af, dart-lang/sdk@349db4e, dart-lang/sdk@a92866a, and dart-lang/sdk@24d44be.

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Added Fixed label.