Add asserts to String::GetFlatContent.

src/json-stringifier.h

 // 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 761 matching lines @@
           vector.start(),
           SeqOneByteString::cast(*current_part_)->GetChars() + current_index_,
           length);
     } else {
       current_index_ += SerializeStringUnchecked_(
           vector.start(),
           SeqTwoByteString::cast(*current_part_)->GetChars() + current_index_,
           length);
     }
   } else {
-    String* string_location = *string;
-    Vector<const Char> vector = GetCharVector<Char>(string);
+    String* string_location = NULL;
+    Vector<const Char> vector(NULL, 0);
     for (int i = 0; i < length; i++) {
+      // If GC moved the string, we need to refresh the vector.
+      if (*string != string_location) {
+        AssertNoAllocation no_gc;
+        // This does not actually prevent the string from being relocated later.
+        vector = GetCharVector<Char>(string);
+        string_location = *string;
+      }
       Char c = vector[i];
       if (DoNotEscape(c)) {
         Append_<is_ascii, Char>(c);
       } else {
         Append_<is_ascii, uint8_t>(reinterpret_cast<const uint8_t*>(
             &JsonEscapeTable[c * kJsonEscapeTableEntrySize]));
       }
-      // If GC moved the string, we need to refresh the vector.
-      if (*string != string_location) {
-        vector = GetCharVector<Char>(string);
-        string_location = *string;
-      }
     }
   }
 
   Append_<is_ascii, uint8_t>('"');
 }
 
 
 template <>
 bool BasicJsonStringifier::DoNotEscape(uint8_t c) {
   return c >= '#' && c <= '~' && c != '\\';
@@ skipping 17 matching lines @@
 
 template <>
 Vector<const uc16> BasicJsonStringifier::GetCharVector(Handle<String> string) {
   String::FlatContent flat = string->GetFlatContent();
   ASSERT(flat.IsTwoByte());
   return flat.ToUC16Vector();
 }
 
 
 void BasicJsonStringifier::SerializeString(Handle<String> object) {
-  FlattenString(object);
-  String::FlatContent flat = object->GetFlatContent();
+  object = FlattenGetString(object);
   if (is_ascii_) {
-    if (flat.IsAscii()) {
+    if (object->IsOneByteRepresentation()) {
       SerializeString_<true, uint8_t>(object);
     } else {
       ChangeEncoding();
       SerializeString(object);
     }
   } else {
-    if (flat.IsAscii()) {
+    if (object->IsOneByteRepresentation()) {
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_