Changes to improve the performance of reading a snapshot.
Changes to improve the performance of reading a snapshot.

There are two changes:
1. The decoding loop in ReadStream is unrolled.
2. Small reorganizaiton of PageSpace::TryAllocate to avoid
   copying SpaceUsuage structs around.

These changes improve performance of CorelibIsolateStartup
~10-12% on ia32.

R=iposva@google.com

Review URL: https://codereview.chromium.org//313393003

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@37141 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk