Cordon the Node Sass API off into a legacy directory (#1501)

lib/src/async_compile.dart

@@ -11,7 +11,7 @@ import 'async_import_cache.dart';
 import 'callable.dart';
 import 'compile_result.dart';
 import 'importer.dart';
-import 'importer/node.dart';
+import 'importer/legacy_node.dart';
 import 'io.dart';
 import 'logger.dart';
 import 'logger/terse.dart';

lib/src/compile.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_compile.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 1a1251aa9f7312612a64760f59803568bd09a07c
+// Checksum: f8b5bf7eafbe3523ca4df1a6832e131c5c03986b
 //
 // ignore_for_file: unused_import
 
@@ -20,7 +20,7 @@ import 'import_cache.dart';
 import 'callable.dart';
 import 'compile_result.dart';
 import 'importer.dart';
-import 'importer/node.dart';
+import 'importer/legacy_node.dart';
 import 'io.dart';
 import 'logger.dart';
 import 'logger/terse.dart';

lib/src/importer/node.dart → lib/src/importer/legacy_node.dart

@@ -2,4 +2,4 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-export 'node/interface.dart' if (dart.library.js) 'node/implementation.dart';
+export 'legacy_node/interface.dart' if (dart.library.js) 'legacy_node/implementation.dart';

lib/src/importer/node/implementation.dart → lib/src/importer/legacy_node/implementation.dart

@@ -10,10 +10,10 @@ import 'package:tuple/tuple.dart';
 
 import '../../io.dart';
 import '../../node/function.dart';
-import '../../node/importer_result.dart';
+import '../../node/legacy/importer_result.dart';
+import '../../node/legacy/render_context.dart';
 import '../../node/utils.dart';
 import '../../util/nullable.dart';
-import '../../node/render_context.dart';
 import '../utils.dart';
 
 /// An importer that encapsulates Node Sass's import logic.