Avoid the creation of a string builder for joining one-element arrays.

src/array.js

 // Copyright 2006-2008 Google Inc. 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 93 matching lines @@
     // visited arrays.
     if (!%PushIfAbsent(visited_arrays, array)) return '';
   }
 
   // Attempt to convert the elements.
   try {
     if (UseSparseVariant(array, length, is_array) && separator === '') {
       return SparseJoin(array, length, convert);
     }
 
+    // Fast case for one-element arrays.
+    if (length === 1) {
+      var e = array[0];
+      if (!IS_UNDEFINED(e) || (0 in array)) {
+        return convert(e);
+      }
+    }
+
     var builder = new StringBuilder();
 
     for (var i = 0; i < length; i++) {
       var e = array[i];
       if (i != 0) builder.add(separator);
       if (!IS_UNDEFINED(e) || (i in array)) {
         builder.add(convert(e));
       }
     }
     return builder.generate();
@@ skipping 787 matching lines @@
     ArrayEvery: 1,
     ArrayMap: 1,
     ArrayIndexOf: 1,
     ArrayLastIndexOf: 1,
     ArrayPush: 1
   });
 };
 
 
 SetupArray();