Add internal operation that converts a growable list to a fixed list.

runtime/lib/growable_array.cc

Index: runtime/lib/growable_array.cc
diff --git a/runtime/lib/growable_array.cc b/runtime/lib/growable_array.cc
index 5cbf58537b873429ba27668111357b191b34fd9c..ccf86a54209decc0cc5e72691da87add79cc4616 100644
--- a/runtime/lib/growable_array.cc
+++ b/runtime/lib/growable_array.cc
@@ -91,4 +91,11 @@ DEFINE_NATIVE_ENTRY(GrowableList_setData, 2) {
   return Object::null();
 }
 
+
+DEFINE_NATIVE_ENTRY(GrowableList_toFixedLength, 1) {
+  GET_NON_NULL_NATIVE_ARGUMENT(GrowableObjectArray, array,
+                               arguments->NativeArgAt(0));
+  return Array::MakeArray(array);
+}
+
 }  // namespace dart