Don't touch files whose contents do not change. Fix #127.

lib/src/io.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.io;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 
@@ skipping 20 matching lines @@
   }
 }
 
 /// Runs the formatter on [file].
 void processFile(File file, {String label, bool overwrite, int pageWidth}) {
   if (label == null) label = file.path;
   if (overwrite == null) overwrite = false;
 
   var formatter = new DartFormatter(pageWidth: pageWidth);
   try {
-    var output = formatter.format(file.readAsStringSync(), uri: file.path);
+    var source = file.readAsStringSync();
+    var output = formatter.format(source, uri: file.path);
     if (overwrite) {
-      file.writeAsStringSync(output);
-      print("Formatted $label");
+      if (source != output) {
+        file.writeAsStringSync(output);
+        print("Formatted $label");
+      } else {
+        print("Unchanged $label");
+      }
     } else {
       // Don't add an extra newline.
       stdout.write(output);
     }
   } on FormatterException catch (err) {
     stderr.writeln(err.message());
   } catch (err, stack) {
     stderr.writeln('''Hit a bug in the formatter when formatting $label
   Please report at: github.com/dart-lang/dart_style/issues
 $err
 $stack''');
   }
 }