[es6] correct constant_elements backing-store when spread is used

src/ast.cc

Index: src/ast.cc
diff --git a/src/ast.cc b/src/ast.cc
index f7eab6ec710dd438fbe3d9095e90f5e00e086d85..5d76efd6be2c4f090af6e38e92fe25022d634ce3 100644
--- a/src/ast.cc
+++ b/src/ast.cc
@@ -548,6 +548,9 @@ void ArrayLiteral::BuildConstantElements(Isolate* isolate) {
   }
   JSObject::ValidateElements(array);
   Handle<FixedArrayBase> element_values(array->elements());
+  if (array_index != values()->length()) {
+    element_values->set_length(array_index);
+  }
 
   // Simple and shallow arrays can be lazily copied, we transform the
   // elements array to a copy-on-write array.