Stricter String->IsFlat.

src/objects-inl.h

 // Copyright 2006-2008 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 1303 matching lines @@
 }
 
 
 StringRepresentationTag String::map_representation_tag(Map* map) {
   uint32_t tag = map->instance_type() & kStringRepresentationMask;
   return static_cast<StringRepresentationTag>(tag);
 }
 
 
 bool String::IsFlat() {
-  String* current = this;
-  while (true) {
-    switch (current->representation_tag()) {
-      case kConsStringTag:
-        return String::cast(ConsString::cast(current)->second())->length() == 0;
-      case kSlicedStringTag:
-        current = String::cast(SlicedString::cast(this)->buffer());
-        break;
-      default:
-        return true;
+  switch (this->representation_tag()) {
+    case kConsStringTag:
+      // Only flattened strings have second part empty.
+      return String::cast(ConsString::cast(this)->second())->length() == 0;
+    case kSlicedStringTag: {
+      String* slice = String::cast(SlicedString::cast(this)->buffer());
+      StringRepresentationTag tag = slice->representation_tag();
+      return tag == kSeqStringTag || tag == kExternalStringTag;
     }
+    default:
+      return true;
   }
 }
 
 
 uint16_t AsciiString::AsciiStringGet(int index) {
   ASSERT(index >= 0 && index < length());
   return READ_BYTE_FIELD(this, kHeaderSize + index * kCharSize);
 }
 
 
@@ skipping 834 matching lines @@
 #undef WRITE_INT_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_