Add experimental support for external two-byte symbols....

include/v8.h

Index: include/v8.h
===================================================================
--- include/v8.h	(revision 766)
+++ include/v8.h	(working copy)
@@ -1811,7 +1811,7 @@
 };
 
 
-// --- C o u n t e r s  C a l l b a c k s
+// --- C o u n t e r s  C a l l b a c k s ---
 
 typedef int* (*CounterLookupCallback)(const wchar_t* name);
 
@@ -1832,9 +1832,24 @@
 typedef void (*GCCallback)();
 
 
-//  --- C o n t e x t  G e n e r a t o r
+// --- E x t e r n a l  S y m b o l  C a l l b a c k ---
 
 /**
+ * Callback used to allocate certain V8 symbols as external strings.
+ *
+ * Allocations are not allowed in the callback function, you therefore
+ * cannot manipulate objects (set or delete properties for example)
+ * since it is possible such operations will result in the allocation
+ * of objects.
+ */
+typedef String::ExternalStringResource* (*ExternalSymbolCallback)(
+    const char* chars,
+    size_t length);
+
+
+// --- C o n t e x t  G e n e r a t o r ---
+
+/**
  * Applications must provide a callback function which is called to generate
  * a context if a context was not deserialized from the snapshot.
  */
@@ -1932,6 +1947,20 @@
   static void SetGlobalGCEpilogueCallback(GCCallback);
 
   /**
+   * Applications can register a callback that will be used when
+   * allocating most of the V8 symbols.  The callback must return an
+   * external string resource that represents the symbols.
+   *
+   * Most often when performing a property lookup the key will be a
+   * symbol.  Allocating symbols as external strings can reduce the
+   * amount of string conversions needed when using interceptors and
+   * accessors.
+   *
+   * \note This is an experimental feature and it might be removed.
+   */
+  static void SetExternalSymbolCallback(ExternalSymbolCallback);
+
+  /**
    * Allows the host application to group objects together. If one
    * object in the group is alive, all objects in the group are alive.
    * After each garbage collection, object groups are removed. It is