Added error handling and a better error message if we fail to write output in docgen.

pkg/docgen/lib/src/generator.dart

 // Copyright (c) 2014, 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.
 
 library docgen.generator;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:convert';
 import 'dart:io';
@@ skipping 118 matching lines @@
 
 /// Writes [text] to a file in the output directory.
 void _writeToFile(String text, String filename) {
   if (text == null) return;
 
   var filePath = path.join(_outputDirectory, filename);
 
   var parentDir = new Directory(path.dirname(filePath));
   if (!parentDir.existsSync()) parentDir.createSync(recursive: true);
 
-  new File(filePath)
-      .writeAsStringSync(text, mode: FileMode.WRITE);
+  try {
+    new File(filePath)
+        .writeAsStringSync(text, mode: FileMode.WRITE);
+  } on FileSystemException catch (e) {
+    print('Failed to write to the path $filePath. Do you have write '
+        'permissions to that directory? If not, please specifi a different '

[kevmoo, 2014/05/09 00:24:02]

specify

+        'output directory using the --out option.');
+    exit(1);
+  }
 }
 
 /// Resolve all the links in the introductory comments for a given library or
 /// package as specified by [filename].
 String _readIntroductionFile(String fileName, bool includeSdk) {
   var linkResolver = (name) => globalFixReference(name);
   var defaultText = includeSdk ? _DEFAULT_SDK_INTRODUCTION : '';
   var introText = defaultText;
   if (fileName.isNotEmpty) {
     var introFile = new File(fileName);
@@ skipping 304 matching lines @@
   * [Samples](http://www.dartlang.org/samples/)
   * [A Tour of the Dart Libraries](http://www.dartlang.org/docs/dart-up-and-running/contents/ch03.html)
 
 This API reference is automatically generated from the source code in the
 [Dart project](https://code.google.com/p/dart/).
 If you'd like to contribute to this documentation, see
 [Contributing](https://code.google.com/p/dart/wiki/Contributing)
 and
 [Writing API Documentation](https://code.google.com/p/dart/wiki/WritingApiDocumentation).
 """;