Verify that GC can handle trailing fillers in large pages.

Verify that GC can handle trailing fillers in large pages.

Assert that filler objects created by String::MakeExternal and Array::MakeArray in large pages are never marked.

Add unit test that exercises the verification code.

R=rmacnak@google.com

Committed: https://code.google.com/p/dart/source/detail?r=37577

[rmacnak, 2014-06-21 00:57:42]

LGTM

[Ivan Posva, 2014-06-21 05:17:37]

Daniel,

How did you run across this issue? I don't think the filler objects were ever
marked, correct?

Thanks,
-Ivan

[koda, 2014-06-22 00:42:57]

On 2014/06/21 05:17:37, Ivan Posva wrote:
> Daniel,
> 
> How did you run across this issue? I don't think the filler objects were ever
> marked, correct?
> 
> Thanks,
> -Ivan

I just realized this when contemplating how to display large object pages in the
observatory.

As far as I can tell, this has not bitten us yet, since the fillers are
(fortunately) always *after* the live object.

Still, I wanted to proactively add the check and document this risky special
case.

[koda, 2014-06-22 00:49:44]

Committed patchset #2 manually as r37577 (presubmit successful).