Revert "Make toLocaleString on arrays always call toLocaleString on its elements."

src/js/array.js

Index: src/js/array.js
diff --git a/src/js/array.js b/src/js/array.js
index 2fde9fb3d4b3ae3f4d9f3abc849537c7f63fb339..c29b8f7256ad1d2c568f5a5103d4c666f4aaf485 100644
--- a/src/js/array.js
+++ b/src/js/array.js
@@ -151,7 +151,8 @@ function DoJoin(array, length, is_array, separator, convert) {
 
   // Fast case for one-element arrays.
   if (length === 1) {
-    return convert(array[0]);
+    var e = array[0];
+    return IS_STRING(e) ? e : convert(e);
   }
 
   // Construct an array for the elements.
@@ -160,14 +161,31 @@ function DoJoin(array, length, is_array, separator, convert) {
   // We pull the empty separator check outside the loop for speed!
   if (separator === '') {
     for (var i = 0; i < length; i++) {
-      elements[i] = convert(array[i]);
+      var e = array[i];
+      elements[i] = IS_STRING(e) ? e : convert(e);
     }
     return %StringBuilderConcat(elements, length, '');
   }
   // Non-empty separator case.
-  elements[0] = convert(array[0]);
-  for (var i = 1; i < length; i++) {
-    elements[i] = convert(array[i]);
+  // If the first element is a number then use the heuristic that the
+  // remaining elements are also likely to be numbers.
+  var e = array[0];
+  if (IS_NUMBER(e)) {
+    elements[0] = %_NumberToString(e);
+    for (var i = 1; i < length; i++) {
+      e = array[i];
+      if (IS_NUMBER(e)) {
+        elements[i] = %_NumberToString(e);
+      } else {
+        elements[i] = IS_STRING(e) ? e : convert(e);
+      }
+    }
+  } else {
+    elements[0] = IS_STRING(e) ? e : convert(e);
+    for (var i = 1; i < length; i++) {
+      e = array[i];
+      elements[i] = IS_STRING(e) ? e : convert(e);
+    }
   }
   return %StringBuilderJoin(elements, length, separator);
 }