Generate Sky's _sdkext file using the build system

mojo/public/dart/rules.gni

 # 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.
 
 # Rules to generate zipped applications for Dart.
 # Rules to generate dart-pkg and dart-pkg/packages.
 
 import("//build/module_args/mojo.gni")
 
 template("dartzip_package") {
@@ skipping 209 matching lines @@
 #       Note: this can only contain mojom targets.
 #
 #   datadeps (optional)
 #
 #   sdk_ext_directory (optional)
 #       Directory containing sdk-ext .dart sources.
 #
 #   sdk_ext_files (optional)
 #       List of sources to include in sdk-ext.
 #
+#   sdk_ext_mappings (optional)
+#       Mappings for dart libraries that are part of of sdk_ext.
+#
 template("dart_pkg") {
   if (defined(invoker.pkg_dir)) {
     pubspec_yaml_path = rebase_path("pubspec.yaml", "", invoker.pkg_dir)
   } else {
     pubspec_yaml_path = rebase_path("pubspec.yaml")
   }
   dart_package_name_script =
       rebase_path("mojo/public/tools/dart_package_name.py", ".", mojo_sdk_root)
   package_name = exec_script(dart_package_name_script,
                              [
@@ skipping 37 matching lines @@
     sdk_ext_directory = []
     if (defined(invoker.sdk_ext_directory)) {
       sdk_ext_directory += [ invoker.sdk_ext_directory ]
     }
 
     sdk_ext_files = []
     if (defined(invoker.sdk_ext_files)) {
       sdk_ext_files += invoker.sdk_ext_files
     }
 
+    sdk_ext_mappings = []
+    if (defined(invoker.sdk_ext_mappings)) {
+      sdk_ext_mappings += invoker.sdk_ext_mappings
+    }
+
     script = rebase_path("mojo/public/tools/dart_pkg.py", ".", mojo_sdk_root)
     outputs = [
       stamp_file,
     ]
 
     if (defined(invoker.sources)) {
       sources = invoker.sources
     } else {
       assert(defined(invoker.pkg_dir))
       list_script = rebase_path("build/ls.py", ".", mojo_sdk_root)
@@ skipping 18 matching lines @@
              "--pkg-directory",
              pkg_directory,
              "--package-root",
              package_root,
              "--stamp-file",
              rebase_path(stamp_file),
              "--package-sources",
            ] + rebase_path(sources) + [ "--mojom-sources" ] +
            rebase_path(mojom_sources, "", mojo_sdk_root) +
            [ "--sdk-ext-directories" ] + rebase_path(sdk_ext_directory) +
-           [ "--sdk-ext-files" ] + rebase_path(sdk_ext_files)
+           [ "--sdk-ext-files" ] + rebase_path(sdk_ext_files) +
+           [ "--sdk-ext-mappings" ] + sdk_ext_mappings
   }
 }