Fix to ensure only modified files are written to (dartbug.com/16093).

pkg/analyzer/bin/formatter.dart

 #!/usr/bin/env dart
 
 // Copyright (c) 2013, 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 'dart:convert';
 import 'dart:io';
 
 import 'package:args/args.dart';
@@ skipping 95 matching lines @@
   if (_isDartFile(file)) {
     if (_isPatchFile(file) && !paths.contains(file.path)) {
       _log('Skipping patch file "${file.path}"');
       return;
     }
     try {
       var buffer = new StringBuffer();
       var rawSource = file.readAsStringSync();
       var formatted = _format(rawSource, CodeKind.COMPILATION_UNIT);
       if (overwriteFileContents) {
-        file.writeAsStringSync(formatted);
+        // Only touch files files whose contents will be changed
+        if (rawSource != formatted) {
+          file.writeAsStringSync(formatted);
+        }
       } else {
         print(formatted);
       }
     } catch (e) {
       _log('Unable to format "${file.path}": $e');
     }
   }
 }
 
 _isPatchFile(file) => file.path.endsWith('_patch.dart');
@@ skipping 69 matching lines @@
                      'offset': formatResult.selection.offset,
                      'length': formatResult.selection.length
                   }
     });
 
 /// Log the given [msg].
 _log(String msg) {
   //TODO(pquitslund): add proper log support
   print(msg);
 }