Make it possible for sky-element to register any tag name

sky/engine/bindings/BUILD.gn

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//sky/engine/bindings/bindings.gni")
 import("//sky/engine/core/core.gni")
 
 source_set("bindings") {
   sources = [
     "builtin.cc",
     "builtin.h",
     "builtin_natives.cc",
     "builtin_natives.h",
     "builtin_sky.cc",
     "builtin_sky.h",
+    "custom/dart_element_custom.cc",
     "dart_callback.cc",
     "dart_callback.h",
     "dart_event_listener.cc",
     "dart_event_listener.h",
     "exception_messages.cc",
     "exception_messages.h",
     "exception_state.cc",
     "exception_state.h",
     "exception_state_placeholder.cc",
     "exception_state_placeholder.h",
@@ skipping 363 matching lines @@
   ]
 
   include_dirs = [
     "..",
     "$root_build_dir",
   ]
 
   sources = get_target_outputs(":compile_idls")
   sources += get_target_outputs(":generate_dart_globals")
 }