Fix crash due RegExpAtom method called on RegExpCharacterClass object.

src/ast.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 944 matching lines @@
   for (int i = 0; i < chardata.length(); i++) {
     stream()->Add("%k", chardata[i]);
   }
   stream()->Add("'");
   return NULL;
 }
 
 
 void* RegExpUnparser::VisitText(RegExpText* that, void* data) {
   if (that->elements()->length() == 1) {
-    that->elements()->at(0).data.u_atom->Accept(this, data);
+    that->elements()->at(0).tree()->Accept(this, data);
   } else {
     stream()->Add("(!");
     for (int i = 0; i < that->elements()->length(); i++) {
       stream()->Add(" ");
-      that->elements()->at(i).data.u_atom->Accept(this, data);
+      that->elements()->at(i).tree()->Accept(this, data);
     }
     stream()->Add(")");
   }
   return NULL;
 }
 
 
 void* RegExpUnparser::VisitQuantifier(RegExpQuantifier* that, void* data) {
   stream()->Add("(# %i ", that->min());
   if (that->max() == RegExpTree::kInfinity) {
@@ skipping 210 matching lines @@
     OS::SNPrintF(buffer, "%d", Smi::cast(*value_)->value());
     str = arr;
   } else {
     str = DoubleToCString(value_->Number(), buffer);
   }
   return factory->NewStringFromAscii(CStrVector(str));
 }
 
 
 } }  // namespace v8::internal