5% smaller snapshots by omitting object ids when possible

Object ids in snapshots are assigned sequentially, and can in many cases be omitted by the writer and inferred by the reader. The exceptions are the classes that are serialized in two steps (arrays and user-level Dart instances), where the second step must include the id.

For standalone, makes full snapshot 5% smaller, and startup ~1.5% faster when measured on a Nexus 10.

R=zra@google.com

Committed: https://code.google.com/p/dart/source/detail?r=38261

[zra, 2014-07-15 16:50:30]

lgtm

https://codereview.chromium.org/387993007/diff/80001/runtime/vm/snapshot_ids.h
File runtime/vm/snapshot_ids.h (right):

https://codereview.chromium.org/387993007/diff/80001/runtime/vm/snapshot_ids....
runtime/vm/snapshot_ids.h:23: kOmittedObjectId,
Would it make sense to hardcode this to a value so that we are sure the header
fits in a byte? (2 bits of tag, 6 bits of data.)

[koda, 2014-07-15 21:29:12]

On 2014/07/15 16:50:30, zra wrote:
> lgtm
> 
> https://codereview.chromium.org/387993007/diff/80001/runtime/vm/snapshot_ids.h
> File runtime/vm/snapshot_ids.h (right):
> 
>
https://codereview.chromium.org/387993007/diff/80001/runtime/vm/snapshot_ids....
> runtime/vm/snapshot_ids.h:23: kOmittedObjectId,
> Would it make sense to hardcode this to a value so that we are sure the header
> fits in a byte? (2 bits of tag, 6 bits of data.)

Added a unit test that expects a single-byte encoding for an omitted-id header.

[koda, 2014-07-15 21:47:05]

Committed patchset #6 manually as r38261 (presubmit successful).