Support deploying an npm package exporting a formatCode method.

tool/node_format_service.dart

+// 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.
+
+import 'package:js/js.dart';
+
+import 'package:dart_style/dart_style.dart';
+
+@JS()
+@anonymous
+class FormatResult {
+  external factory FormatResult({String code, String error});
+  external String get code;
+  external String get error;
+}
+
+@JS('exports.formatCode')
+external set formatCode(Function formatter);
+
+void main() {
+  formatCode = allowInterop((String source) {
+    var formatter = new DartFormatter();
+    try {
+      return new FormatResult(
+          code: new DartFormatter().format(source));
+    } on FormatterException {
+      // Do nothing.
+    }
+
+    // Maybe it's a statement.
+    try {
+      return new FormatResult(
+          code: formatter.formatStatement(source));
+    } on FormatterException catch (err) {
+      return new FormatResult(code: source, error: "$err");
+    }
+  });
+}