Revert of [fullcodegen] Remove the hacky %_FastOneByteArrayJoin intrinsic.

src/js/array.js

Index: src/js/array.js
diff --git a/src/js/array.js b/src/js/array.js
index b8cf3ff5d222a3fb65266a9539890b218b1b30d4..28d964eaae87a36e05585a8755938a5fb3feb0d0 100644
--- a/src/js/array.js
+++ b/src/js/array.js
@@ -209,6 +209,8 @@
         elements[elements_length++] = e;
       }
       elements.length = elements_length;
+      var result = %_FastOneByteArrayJoin(elements, '');
+      if (!IS_UNDEFINED(result)) return result;
       return %StringBuilderConcat(elements, elements_length, '');
     }
     // Non-empty separator case.
@@ -231,6 +233,9 @@
         elements[i] = e;
       }
     }
+    var result = %_FastOneByteArrayJoin(elements, separator);
+    if (!IS_UNDEFINED(result)) return result;
+
     return %StringBuilderJoin(elements, length, separator);
   } finally {
     // Make sure to remove the last element of the visited array no
@@ -441,6 +446,9 @@
   } else {
     separator = TO_STRING(separator);
   }
+
+  var result = %_FastOneByteArrayJoin(array, separator);
+  if (!IS_UNDEFINED(result)) return result;
 
   // Fast case for one-element arrays.
   if (length === 1) {