Dart: Rebase generated imports from the SDK to the SDK base directory.

mojo/public/interfaces/application/BUILD.gn

 # Copyright 2014 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("../../tools/bindings/mojom.gni")
 
 # GYP version: mojo/public/mojo_public.gyp:mojo_application_bindings
 mojom("application") {
   sources = [
     "application.mojom",
     "service_provider.mojom",
     "shell.mojom",
   ]
 
-  # This base dir ensures that Dart's Mojo SDK can be imported with, e.g.,
+  # The import_from ensures that Dart's Mojo SDK can be imported with, e.g.,
   # import 'package:mojo/public/dart/core.dart' even when the Mojo SDK lives
   # somewhere else in the source tree.
-  base_dir = mojo_root
+  import_from = mojo_root
 }