GN: Relativize occurrances of //dart

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.
 
+default_dart_root = rebase_path("../..")
+
 # 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".
+#       Path to the Dart SDK source root. Default value is "../..".
 #
 # 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")
+    dart_root = default_dart_root
   }
   if (defined(invoker.destination)) {
     destination = invoker.destination
   } else {
     destination = "$root_gen_dir/dart_sdk"
   }
   dart_sdk_sdk_lib_path =
       rebase_path("sdk/lib", "", dart_root)
   dart_sdk_tools_gypi_to_gn_path =
       rebase_path("tools/gypi_to_gn.py", "", dart_root)
   # The name of the SDK library being copied.
   lib_name = invoker.sdk_lib_name
   # The path to the libraries source directory.
   lib_path = rebase_path(lib_name, "", dart_sdk_sdk_lib_path)
   # The path to the sources gypi.
   lib_sources_gypi = lib_name + "_sources.gypi"
   # Get the contents of the gypi file.
   sdk_lib_sources_gypi =
       exec_script(dart_sdk_tools_gypi_to_gn_path,
                   [rebase_path(lib_sources_gypi, "", lib_path)],
                   "scope",
                   [rebase_path(lib_sources_gypi, "", lib_path)])
   copy(target_name) {
     sources = rebase_path(sdk_lib_sources_gypi.sources, "", lib_path)
     outputs = [ "$destination/$lib_name/{{source_file_part}}" ]
   }
 }