Gracefully handle IO errors when failing to overwrite a file.

lib/src/formatter_options.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.src.formatter_options;
 
 import 'dart:convert';
 import 'dart:io';
 
 import 'source_code.dart';
@@ skipping 98 matching lines @@
         "length": output.selectionLength != null ? output.selectionLength : -1
       }
     }));
   }
 }
 
 /// Overwrites each file with its formatted result.
 class _OverwriteReporter extends _PrintReporter {
   void afterFile(File file, String label, SourceCode output, {bool changed}) {
     if (changed) {
-      file.writeAsStringSync(output.text);
-      print("Formatted $label");
+      try {
+        file.writeAsStringSync(output.text);
+        print("Formatted $label");
+      } on FileSystemException catch (err) {
+        stderr.writeln("Could not overwrite $label: "
+            "${err.osError.message} (error code ${err.osError.errorCode})");
+      }
     } else {
       print("Unchanged $label");
     }
   }
 }
 
 /// A decorating reporter that reports how long it took for format each file.
 class ProfileReporter implements OutputReporter {
   final OutputReporter _inner;
 
@@ skipping 59 matching lines @@
     var elapsed = new DateTime.now().difference(_ongoing.remove(label));
     if (elapsed.inMilliseconds >= 10) {
       _elapsed[label] = elapsed;
     } else {
       _elided++;
     }
 
     _inner.afterFile(file, label, output, changed: changed);
   }
 }