Don't use AddSubstring for external natives

src/utils.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_UTILS_H_
 #define V8_UTILS_H_
 
 #include <limits.h>
 #include <stdlib.h>
 #include <string.h>
@@ skipping 845 matching lines @@
   void AddCharacter(char c) {
     DCHECK(c != '\0');
     DCHECK(!is_finalized() && position_ < buffer_.length());
     buffer_[position_++] = c;
   }
 
   // Add an entire string to the builder. Uses strlen() internally to
   // compute the length of the input string.
   void AddString(const char* s);
 
-  // Add the first 'n' characters of the given string 's' to the
+  // Add the first 'n' characters of the given 0-terminated string 's' to the
   // builder. The input string must have enough characters.
   void AddSubstring(const char* s, int n);
 
   // Add character padding to the builder. If count is non-positive,
   // nothing is added to the builder.
   void AddPadding(char c, int count);
 
   // Add the decimal representation of the value.
   void AddDecimalInteger(int value);
 
@@ skipping 660 matching lines @@
   // Takes the address of the limit variable in order to find out where
   // the top of stack is right now.
   uintptr_t limit = reinterpret_cast<uintptr_t>(&limit);
   return limit;
 }
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_UTILS_H_