Change SmartMove no-op behavior to match SimpleMove (and ES6 spec)

src/array.js

Index: src/array.js
diff --git a/src/array.js b/src/array.js
index 9e83d979e09b5f31c034cc457a32d62d809e0b15..585d1da8427507f0ef0ea3cb12470d3c9bc21488 100644
--- a/src/array.js
+++ b/src/array.js
@@ -234,6 +234,8 @@ function SmartSlice(array, start_i, del_count, len, deleted_elements) {
 // This function implements the optimized splice implementation that can use
 // special array operations to handle sparse arrays in a sensible fashion.
 function SmartMove(array, start_i, del_count, len, num_additional_args) {
+  // Bail out if no moving is necessary.
+  if (num_additional_args === del_count) return;
   // Move data to new array.
   var new_array = new InternalArray(len - del_count + num_additional_args);
   var indices = %GetArrayKeys(array, len);