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

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 766)
+++ src/heap.h	(working copy)
@@ -356,10 +356,18 @@
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.
   // Please note this function does not perform a garbage collection.
-  static Object* AllocateSymbol(unibrow::CharacterStream* buffer,
-                                int chars,
-                                uint32_t length_field);
+  static inline Object* AllocateSymbol(Vector<const char> str,
+                                       int chars,
+                                       uint32_t length_field);
 
+  static Object* AllocateInternalSymbol(unibrow::CharacterStream* buffer,
+                                        int chars,
+                                        uint32_t length_field);
+
+  static Object* AllocateExternalSymbol(Vector<const char> str,
+                                        int chars);
+
+
   // Allocates and partially initializes a String.  There are two String
   // encodings: ASCII and two byte.  These functions allocate a string of the
   // given length and set its map and length fields.  The characters of the
@@ -527,6 +535,8 @@
       ExternalAsciiString::Resource* resource);
   static Object* AllocateExternalStringFromTwoByte(
       ExternalTwoByteString::Resource* resource);
+  static Object* AllocateExternalSymbolFromTwoByte(
+      ExternalTwoByteString::Resource* resource);
 
   // Allocates an uninitialized object.  The memory is non-executable if the
   // hardware and OS allow.
@@ -595,6 +605,10 @@
     global_gc_epilogue_callback_ = callback;
   }
 
+  static void SetExternalSymbolCallback(ExternalSymbolCallback callback) {
+    global_external_symbol_callback_ = callback;
+  }
+
   // Heap roots
 #define ROOT_ACCESSOR(type, name) static type* name() { return name##_; }
   ROOT_LIST(ROOT_ACCESSOR)
@@ -854,6 +868,9 @@
   static GCCallback global_gc_prologue_callback_;
   static GCCallback global_gc_epilogue_callback_;
 
+  // Callback function used for allocating external symbols.
+  static ExternalSymbolCallback global_external_symbol_callback_;
+
   // Checks whether a global GC is necessary
   static GarbageCollector SelectGarbageCollector(AllocationSpace space);