Write barrier audit, part 1

Write barrier audit, part 1.

First step towards complete audit of all writes to the heap for write barrier compliance (at least on simulator debug builds).

This first CL is a semi-mechanical refactoring to route most Object field writes through StoreNonPointer and StoreSmi methods.

A few stores were converted to StorePointer (skipping the write barrier check is premature optimization for those cases).

Next step will be to make raw_ptr() return a const pointer and use the constness to clean up remaining writes in C++.

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40889

[Ivan Posva, 2014-10-02 19:44:46]

LGTM -ip

[koda, 2014-10-02 20:47:52]

Committed patchset #10 (id:180001) manually as r40889 (presubmit successful).