Snapshotted pub executables behave badly with unhandled exceptions

[nex3]

Currently, if a snapshotted pub executable (most commonly one installed with "pub global activate") has an unhandled exception, the executable will be recompiled and re-run three times before ultimately failing. This is du to issue #20802, which makes it impossible for the script running the executable to determine if it's up-to-date without running it, and the fact that the VM emits the same exit code for unhandled exceptions and for invalid snapshot numbers.

[nex3]

Thanks to r43505, I now have out https://codereview.chromium.org/899373002 to fix this.

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Unmarked this as being blocked by #20802.

[nex3]

Fixed in r43737.

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Added Fixed label.

[DartBot]

This issue has been moved to dart-lang/pub#1232.