Ensure that external symbols have hash codes.

src/heap.cc

Index: src/heap.cc
===================================================================
--- src/heap.cc	(revision 778)
+++ src/heap.cc	(working copy)
@@ -2142,6 +2142,8 @@
   // Patch the resource pointer of the result.
   ExternalTwoByteString* result = ExternalTwoByteString::cast(obj);
   result->set_resource(resource);
+  // Force hash code to be computed.
+  result->Hash();
   ASSERT(result->IsEqualTo(string));
   return result;
 }