pub run should save snapshots of dependencies' executables

[nex3]

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.

[nex3]

r39424 mostly implements this, but there are a few details I want to improve before closing out the bug.

[DartBot]

This comment was originally written by @zoechi

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

[DartBot]

This comment was originally written by @zoechi

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Performance is fine now !

[nex3]

Fixed by r39424, r39431, r39441, and r39528.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#1087.