Use a private own symbol instead of a hidden property for hash codes

src/collection.js

Index: src/collection.js
diff --git a/src/collection.js b/src/collection.js
index db30546165119e53f0c4e03a1b5e856259927f10..97669d9161ba65a145f93e6cf01facd400d35a61 100644
--- a/src/collection.js
+++ b/src/collection.js
@@ -70,6 +70,7 @@ function ComputeIntegerHash(key, seed) {
 }
 %SetInlineBuiltinFlag(ComputeIntegerHash);
 
+var hash_code_symbol = GLOBAL_PRIVATE("hash_code_symbol");
 
 function GetHash(key) {
   if (%_IsSmi(key)) {
@@ -81,6 +82,14 @@ function GetHash(key) {
       return field >>> 2 /* Name::kHashShift */;
     }
   }
+  if (IS_SPEC_OBJECT(key)) {

[adamk, 2015/05/14 15:01:27]

You might need a TODO here for handling proxies appropriately.

Also, will this get the wrong value for the JSGlobalProxy?

[Erik Corry, 2015/05/14 15:42:19]

Probably.  What's the deal with the JSGlobalProxy?

[Toon Verwaest, 2015/05/15 12:24:10]

Due to split-object the global proxy points to the current global object, which
is a single object from the POV of the user. The global proxy represents an
iframe, whereas the global object represents a page in the iframe. If the iframe
navigates, the page changes, and hence the link from the global proxy to the
global object.

Access to the global proxy always relays the access to the global object. So if
you do window[hash_code_symbol], you'll get the hash-code from the current page.
That doesn't remain stable across navigation.

After navigation you won't be able to retrieve values associated with the iframe
anymore (unless there's a hash collision between the 2 pages).

+    var hash = key[hash_code_symbol];

[adamk, 2015/05/14 15:01:27]

We've been using a GET_PRIVATE() for this as a matter of style.

+    if (hash == void 0) {

[adamk, 2015/05/14 15:01:27]

IS_UNDEFINED(hash)

+      hash = $intrandom() | 0;
+      key[hash_code_symbol] = hash;

[adamk, 2015/05/14 15:01:27]

SET_PRIVATE()

+    }
+    return hash;
+  }
   return %GenericHash(key);
 }
 %SetInlineBuiltinFlag(GetHash);