Combine %_SubString and %_StringCharAt.

src/messages.js

 // 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 544 matching lines @@
     var sourceUrlPattern =
         /\/\/@[\040\t]sourceURL=[\040\t]*([^\s\'\"]*)[\040\t]*$/gm;
     // Don't reuse lastMatchInfo here, so we create a new array with room
     // for four captures (array with length one longer than the index
     // of the fourth capture, where the numbering is zero-based).
     var matchInfo = new InternalArray(CAPTURE(3) + 1);
     var match =
         %_RegExpExec(sourceUrlPattern, source, sourceUrlPos - 4, matchInfo);
     if (match) {
       this.cachedNameOrSourceURL =
-          SubString(source, matchInfo[CAPTURE(2)], matchInfo[CAPTURE(3)]);
+          %_SubString(source, matchInfo[CAPTURE(2)], matchInfo[CAPTURE(3)]);
     }
   }
   return this.cachedNameOrSourceURL;
 }
 
 
 SetUpLockedPrototype(Script,
   $Array("source", "name", "line_ends", "line_offset", "column_offset",
          "cachedNameOrSourceURL", "hasCachedNameOrSourceURL" ),
   $Array(
@@ skipping 725 matching lines @@
     %SetOverflowedStackTrace(this, void 0);
   }
 
   %DefineOrRedefineAccessorProperty(
       boilerplate, 'stack', getter, setter, DONT_ENUM);
 
   return boilerplate;
 }
 
 var kStackOverflowBoilerplate = SetUpStackOverflowBoilerplate();