Allow hidden properties and implement GetIdentityHash

- Allow hidden properties to be set on any JSObject through the V8 C++ API.
- Use the hidden properties to expose a IdentityHash accessor.

Implementation detail: Currently the hidden properties are accessed through
a named, but not enumerable property on the JSObject. I am planning to submit this now so that work on tying this new API into the Chromium V8
bindings can proceed while I hide the properties entirely.

[iposva, 2009-03-10 00:07:52]

Kasper, Mads,
Please take a look at this initial implementation.

Mike,
Would this solve some of the issues in the V8 bindings?

Thanks,
-Ivan

[Kasper Lund, 2009-03-10 06:31:06]

LGTM. My only real concern is the timing of the commit; I'd prefer not to get
this pushed to trunk as part of the 1.0.4 push that I hope we can land real soon
(blocked on issue 265).

http://codereview.chromium.org/40324/diff/1/4
File src/api.cc (right):

http://codereview.chromium.org/40324/diff/1/4#newcode1950
Line 1950: return v8::Local<v8::Value>();
Couldn't this just be Local<Value>()? There are a lot of v8:: prefix that aren't
strictly needed in other places too.

http://codereview.chromium.org/40324/diff/1/9
File src/heap.h (right):

http://codereview.chromium.org/40324/diff/1/9#newcode198
Line 198: V(hidden_symbol, "...hidden...")                                      
\
I think this way of hiding the hidden properties has to be fixed before we can
land this change on trunk. I have no worries landing it on bleeding_edge though,
but I would prefer it if that could be postponed until we have pushed 1.0.4
(which includes a bunch of compiler changes).

http://codereview.chromium.org/40324/diff/1/6
File src/objects.cc (right):

http://codereview.chromium.org/40324/diff/1/6#newcode5070
Line 5070: if (!this->HasProperty(key)) {
You end up doing two lookups for the hidden properties here. One for the
HasProperty and one for the GetProperty. Maybe this isn't a big deal because
you'll have to rewrite this anyway when you change the way you hide the
properties, but you may want to use a LocalLookup instead.

http://codereview.chromium.org/40324/diff/1/6#newcode5073
Line 5073: if (create_if_needed) {
I might be inclined to change this to an early bailout check instead: if
(!create_if_needed) return Heap::undefined_value()

[Mads Ager (chromium), 2009-03-10 08:23:39]

Overall, this looks good.  I agree with Kasper on the timing.  We need to get
the new compiler pushed to trunk before landing this in bleeding_edge.  Once we
land this in bleeding_edge, we need to make sure that the real hiding of the
property happens fairly quickly so we are not blocked from pushing bleeding_edge
to trunk.

http://codereview.chromium.org/40324/diff/1/3
File include/v8.h (right):

http://codereview.chromium.org/40324/diff/1/3#newcode1077
Line 1077: int GetIdentityHash();
Having two ways of creating a named hidden value makes naming conflicts
possible.  When we get to the point where the hidden properties are actually
hidden, we should specify that the name used for the hash is reserved
(alternatively, we could remove GetIdentityHash and let the embedder do it
himself with the API below).