Change the way we provide custom dart code for IDL bindings.

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",
@@ skipping 316 matching lines @@
   ]
 
   deps = [
     ":compile_idls",
   ]
 }
 
 copy("copy_core_dart_files") {
   sources = core_dart_files
   outputs = [
-    "$bindings_output_dir/{{source_file_part}}",
+    "$bindings_output_dir/Custom{{source_file_part}}",
   ]
 }
 
 action("generate_dart_sky") {
   sources = core_idl_files + core_dart_files
   script = "$bindings_scripts_dir/compiler.py"
 
   file_list = "$target_gen_dir/${target_name}_file_list.txt"
   write_file(file_list, rebase_path(sources, root_build_dir))
 
@@ skipping 48 matching lines @@
   ]
 
   include_dirs = [
     "..",
     "$root_build_dir",
   ]
 
   sources = get_target_outputs(":compile_idls")
   sources += get_target_outputs(":generate_dart_globals")
 }