Snapshot format is not platform independent

[peter-ahe-google]

As we discovered in issue #6233, the snapshot format is not platform independent.

I'm told that is a bug.

[a-siva]

Peter, currently the VM sets up the following fields in a dart instance:
instance_size - size of a an instance in bytes
offset - offset in bytes of the field in the instance

Since these are values are recorded in bytes the values are different for a 32 bit VM from a 64 bit VM. Hence a snapshot generated on a 32 bit system does not work when read into a 64 bit VM.

There is a proposal to record these values in words which would make them platform independent. I will use this bug as a means for tracking this proposed changed.

[a-siva]

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

[a-siva]

https://codereview.chromium.org//11421117

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Added Fixed label.