Merge r9917 into 3.5 branch. This fixes issue 103259.

src/runtime.cc

 // Copyright 2011 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 6643 matching lines @@
   RUNTIME_ASSERT(elements_array->HasFastElements());
   CONVERT_NUMBER_CHECKED(uint32_t, array_length, Uint32, args[1]);
   CONVERT_CHECKED(String, separator, args[2]);
   // elements_array is fast-mode JSarray of alternating positions
   // (increasing order) and strings.
   // array_length is length of original array (used to add separators);
   // separator is string to put between elements. Assumed to be non-empty.
 
   // Find total length of join result.
   int string_length = 0;
-  bool is_ascii = true;
+  bool is_ascii = separator->IsAsciiRepresentation();
   int max_string_length = SeqAsciiString::kMaxLength;
   bool overflow = false;
   CONVERT_NUMBER_CHECKED(int, elements_length,
                          Int32, elements_array->length());
   RUNTIME_ASSERT((elements_length & 1) == 0);  // Even length.
   FixedArray* elements = FixedArray::cast(elements_array->elements());
   for (int i = 0; i < elements_length; i += 2) {
     RUNTIME_ASSERT(elements->get(i)->IsNumber());
     CONVERT_CHECKED(String, string, elements->get(i + 1));
     int length = string->length();
@@ skipping 6323 matching lines @@
   } else {
     // Handle last resort GC and make sure to allow future allocations
     // to grow the heap without causing GCs (if possible).
     isolate->counters()->gc_last_resort_from_js()->Increment();
     isolate->heap()->CollectAllGarbage(false);
   }
 }
 
 
 } }  // namespace v8::internal