Gracefully handle IO errors when failing to overwrite a file.

test/io_test.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 dart_style.test.io;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:dart_style/src/io.dart';
@@ skipping 16 matching lines @@
   test('handles directory ending in ".dart"', () {
     d.dir('code.dart', [
       d.file('a.dart', unformattedSource),
     ]).create();
 
     schedule(() {
       var dir = new Directory(d.defaultRoot);
       processDirectory(overwriteOptions, dir);
     }, 'Run formatter.');
 
-    d.dir('code.dart', [d.file('a.dart', formattedSource)]).validate();
+    d.dir('code.dart', [
+      d.file('a.dart', formattedSource),
+    ]).validate();
   });
 
   test("doesn't touch unchanged files", () {
     d.dir('code', [
       d.file('bad.dart', unformattedSource),
       d.file('good.dart', formattedSource),
     ]).create();
 
     modTime(String file) {
       return new File(p.join(d.defaultRoot, 'code', file)).statSync().modified;
@@ skipping 100 matching lines @@
 
     d.dir('code', [
       d.file('a.dart', formattedSource),
       d.dir('linked_dir', [
         d.file('b.dart', formattedSource),
       ])
     ]).validate();
   });
 
   if (!Platform.isWindows) {
+    // TODO(rnystrom): Figure out Windows equivalent of chmod and get this
+    // test running on Windows too.
+    test("reports error if file can not be written", () {
+      d.file('a.dart', unformattedSource).create();
+
+      schedule(() {
+        Process.runSync("chmod", ["-w", p.join(d.defaultRoot, 'a.dart')]);
+      }, 'Make file read-only.');
+
+      schedule(() {
+        var file = new File(p.join(d.defaultRoot, 'a.dart'));
+        processFile(overwriteOptions, file);
+      }, 'Run formatter.');
+
+      // Should not have been formatted.
+      d.file('a.dart', unformattedSource).validate();
+    });
+
     test("doesn't follow file symlinks by default", () {
       d.dir('code').create();
       d.file('target_file.dart', unformattedSource).create();
 
       schedule(() {
         // Create a link to the target file in the code directory.
         new Link(p.join(d.defaultRoot, 'code', 'linked_file.dart'))
             .createSync(p.join(d.defaultRoot, 'target_file.dart'));
       }, 'Create symlinks.');
 
@@ skipping 21 matching lines @@
         var dir = new Directory(p.join(d.defaultRoot, 'code'));
         processDirectory(followOptions, dir);
       }, 'running formatter');
 
       d.dir('code', [
         d.file('linked_file.dart', formattedSource),
       ]).validate();
     });
   }
 }