Create an abstraction for the string type flags so that they can be cached....

src/handles.cc

Index: src/handles.cc
===================================================================
--- src/handles.cc	(revision 654)
+++ src/handles.cc	(working copy)
@@ -117,9 +117,10 @@
 
 
 void FlattenString(Handle<String> string) {
-  if (string->IsFlat()) return;
-  CALL_HEAP_FUNCTION_VOID(string->Flatten());
-  ASSERT(string->IsFlat());
+  StringShape shape(*string);
+  if (string->IsFlat(shape)) return;
+  CALL_HEAP_FUNCTION_VOID(string->Flatten(shape));
+  ASSERT(string->IsFlat(StringShape(*string)));
 }
 
 
@@ -216,7 +217,8 @@
 
 
 Handle<String> SubString(Handle<String> str, int start, int end) {
-  CALL_HEAP_FUNCTION(str->Slice(start, end), String);
+  StringShape shape(*str);
+  CALL_HEAP_FUNCTION(str->Slice(shape, start, end), String);
 }