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

src/heap.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 687 matching lines @@
   // failed.
   // Please note this does not perform a garbage collection.
   MUST_USE_RESULT MaybeObject* AllocateStringFromAscii(
       Vector<const char> str,
       PretenureFlag pretenure = NOT_TENURED);
   MUST_USE_RESULT inline MaybeObject* AllocateStringFromUtf8(
       Vector<const char> str,
       PretenureFlag pretenure = NOT_TENURED);
   MUST_USE_RESULT MaybeObject* AllocateStringFromUtf8Slow(
       Vector<const char> str,
+      int non_ascii_start,
       PretenureFlag pretenure = NOT_TENURED);
   MUST_USE_RESULT MaybeObject* AllocateStringFromTwoByte(
       Vector<const uc16> str,
       PretenureFlag pretenure = NOT_TENURED);
 
   // Allocates a symbol in old space based on the character stream.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.
   // Please note this function does not perform a garbage collection.
   MUST_USE_RESULT inline MaybeObject* AllocateSymbol(Vector<const char> str,
@@ skipping 2096 matching lines @@
   AssertNoAllocation no_alloc;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG || LIVE_OBJECT_LIST
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_