Change snapshot versioning code to use plain strings instead of Dart strings

Change snapshot versioning code to use plain strings instead of Dart strings
- This ensures that the versioning itself is not dependent on the Dart object
  layout
- It should also fix the strange performance regression we noticed in tracer
  because of the allocation of an initial version string in the old space.

R=srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=39708

[siva, 2014-08-29 18:53:54]

asiva@google.com changed reviewers:
+ srdjan@google.com

[srdjan, 2014-08-29 20:00:24]

lgtm

https://codereview.chromium.org/522893002/diff/20001/runtime/vm/snapshot.cc
File runtime/vm/snapshot.cc (right):

https://codereview.chromium.org/522893002/diff/20001/runtime/vm/snapshot.cc#n...
runtime/vm/snapshot.cc:482: // NB: New things are allocated only if we're going
to return an error.
s/NB/Note/

[siva, 2014-08-29 20:29:05]

https://codereview.chromium.org/522893002/diff/20001/runtime/vm/snapshot.cc
File runtime/vm/snapshot.cc (right):

https://codereview.chromium.org/522893002/diff/20001/runtime/vm/snapshot.cc#n...
runtime/vm/snapshot.cc:482: // NB: New things are allocated only if we're going
to return an error.
On 2014/08/29 20:00:24, srdjan wrote:
> s/NB/Note/

Done.

[siva, 2014-08-29 20:39:25]

Committed patchset #3 (id:40001) manually as r39708 (presubmit successful).