Add a version number to snapshots

[iposva-google]

Every time the internal data structures in the VM change we need to invalidate existing snapshots as they are now incompatible.

A simple approach could be to bump the version number for every single official build.

More accurate, but significantly more involved would be to increase the version number whenever the information recorded in the snapshot changes. For example, generate a hash based on the VM internal data structures and core libraries flowing into the snapshot.

[iposva-google]

Set owner to @a-siva.
Added Accepted label.

[fsc8000]

Issue #20485 has been merged into this issue.

[nex3]

For context from issue #20485, this is important for supporting issue #20482 and issue #20483. Can we bump up the priority at all?

[iposva-google]

We'll make sure Zach has some time to work on this.

We might start with being overly conservative: Meaning snapshots will NOT be compatible across builds even if the format has not necessarily changed.

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cc @a-siva.
Set owner to @zanderso.
Added this to the 1.7 milestone.
Removed Priority-Unassigned label.
Added Priority-High label.

[zanderso]

Added Started label.

[a-siva]

https://code.google.com/p/dart/source/detail?r=39612

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Added Fixed label.