Keep track of the first non-ascii word/char to avoid redoing the work.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index a7645e25f1ebf512c306dc6120f5b179f5a3dc01..bace902d4dce87cc3b33c242e774c713acc08a27 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -85,13 +85,16 @@ void PromotionQueue::ActivateGuardIfOnTheSamePage() {
 MaybeObject* Heap::AllocateStringFromUtf8(Vector<const char> str,
                                           PretenureFlag pretenure) {
   // Check for ASCII first since this is the common case.
-  if (String::IsAscii(str.start(), str.length())) {
+  const char* start = str.start();
+  int length = str.length();
+  int non_ascii_start = String::NonAsciiStart(start, length);
+  if (non_ascii_start >= length) {
     // If the string is ASCII, we do not need to convert the characters
     // since UTF8 is backwards compatible with ASCII.
     return AllocateStringFromAscii(str, pretenure);
   }
   // Non-ASCII and we need to decode.
-  return AllocateStringFromUtf8Slow(str, pretenure);
+  return AllocateStringFromUtf8Slow(str, non_ascii_start, pretenure);
 }