Fix Flutter Dart role

sdk/lib/rules.gni

 # Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # This build rule will copy the source for one Dart SDK library.
 #
 # Required arguments:
 #   sdk_lib_name
 #       The name of a Dart SDK library.
 #
 # Optional arguments:
+#   destination
+#       Base path to copy sources. Default value is "$root_gen_dir/dart_sdk".
+#
 #   dart_root
 #       Path to the Dart SDK source root. Default value is "//dart".
 #
-# The sources will be copied into $root_gen_dir/dart_sdk_libs/$sdk_lib_name/.
+# The sources will be copied into $root_gen_dir/dart_sdk/$sdk_lib_name/.
 #
 template("dart_sdk_lib_copy") {
   assert(defined(invoker.sdk_lib_name))
   if (defined(invoker.dart_root)) {
     dart_root = rebase_path(invoker.dart_root)
   } else {
     dart_root = rebase_path("//dart")
   }
+  if (defined(invoker.destination)) {
+    destination = invoker.destination
+  } else {
+    destination = "$root_gen_dir/dart_sdk"
+  }
   dart_sdk_sdk_lib_path =
       rebase_path("sdk/lib", "", dart_root)
   dart_sdk_tools_gypi_to_gn_path =
       rebase_path("tools/gypi_to_gn.py", "", dart_root)
   # The name of the SDK library being copied.
   lib_name = invoker.sdk_lib_name
   # The path to the libraries source directory.
   lib_path = rebase_path(lib_name, "", dart_sdk_sdk_lib_path)
   # The path to the sources gypi.
   lib_sources_gypi = lib_name + "_sources.gypi"
   # Get the contents of the gypi file.
   sdk_lib_sources_gypi =
       exec_script(dart_sdk_tools_gypi_to_gn_path,
                   [rebase_path(lib_sources_gypi, "", lib_path)],
                   "scope",
                   [rebase_path(lib_sources_gypi, "", lib_path)])
   copy(target_name) {
     sources = rebase_path(sdk_lib_sources_gypi.sources, "", lib_path)
-    outputs = [ "$root_gen_dir/dart_sdk_libs/$lib_name/{{source_file_part}}" ]
+    outputs = [ "$destination/$lib_name/{{source_file_part}}" ]
   }
 }