Some string optimizations:

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index d299d92a25737d74e6a86b2000994dabb8de0709..9197466e85e4564cfe29e38e814e4865e5c06a64 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -3926,6 +3926,13 @@ class String: public HeapObject {
   inline bool IsAsciiRepresentation();
   inline bool IsTwoByteRepresentation();
 
+  // Check whether this string is an external two-byte string that in
+  // fact contains only ascii characters.
+  //
+  // Such strings may appear when the embedder prefers two-byte
+  // representations even for ascii data.
+  inline bool IsExternalTwoByteStringWithAsciiChars();
+
   // Get and set individual two byte chars in the string.
   inline void Set(int index, uint16_t value);
   // Get individual two byte char in the string.  Repeated calls