Use the new mechanism for distinguishing Node from web compilation

bin/sass.dart

@@ -3,4 +3,4 @@
 // https://opensource.org/licenses/MIT.
 
 export 'package:sass/src/executable.dart'
-    if (node) 'package:sass/src/node.dart';
+    if (dart.library.js) 'package:sass/src/node.dart';

lib/src/importer/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 (node) 'node/implementation.dart';
+export 'node/interface.dart' if (dart.library.js) 'node/implementation.dart';

lib/src/io.dart

@@ -4,4 +4,4 @@
 
 export 'io/interface.dart'
     if (dart.library.io) 'io/vm.dart'
-    if (node) 'io/node.dart';
+    if (dart.library.js) 'io/node.dart';

lib/src/sync_package_resolver.dart

@@ -1,2 +1,2 @@
 export 'package:package_resolver/package_resolver.dart'
-    if (node) 'sync_package_resolver/node.dart';
+    if (dart.library.js) 'sync_package_resolver/node.dart';

tool/grind.dart

@@ -102,6 +102,7 @@ void _js({@required bool release}) {
   var destination = new File('build/sass.dart.js');
 
   var args = [
+    '--categories=Server',
     '-Dnode=true',
     '-Dversion=$_version',
     '-Ddart-version=$_dartVersion',