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

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';
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:unittest/compact_vm_config.dart';
 
 const _SOURCE = 'void  main()  =>  print("hello") ;';
 const _FORMATTED = 'void main() => print("hello");\n';
@@ skipping 27 matching lines @@
     schedule(() {
       var dir = new Directory(d.defaultRoot);
       processDirectory(dir, overwrite: true);
     }, 'Run formatter.');
 
     d.dir('code.dart', [
       d.file('a.dart', _FORMATTED)
     ]).validate();
   });
 
+  test("doesn't touch unchanged files", () {
+    d.dir('code', [
+      d.file('bad.dart', _SOURCE),
+      d.file('good.dart', _FORMATTED),
+    ]).create();
+
+    modTime(String file) {
+      return new File(p.join(d.defaultRoot, 'code', file)).statSync().modified;
+    }
+
+    var badBefore;
+    var goodBefore;
+
+    schedule(() {
+      badBefore = modTime('bad.dart');
+      goodBefore = modTime('good.dart');
+
+      // Wait a bit so the mod time of a formatted file will be different.
+      return new Future.delayed(new Duration(seconds: 1));
+    });
+
+    schedule(() {
+      var dir = new Directory(p.join(d.defaultRoot, 'code'));
+      processDirectory(dir, overwrite: true);
+
+      // Should be touched.
+      var badAfter = modTime('bad.dart');
+      expect(badAfter, isNot(equals(badBefore)));
+
+      // Should not be touched.
+      var goodAfter = modTime('good.dart');
+      expect(goodAfter, equals(goodBefore));
+    });
+  });
+
   test("doesn't follow directory symlinks by default", () {
     d.dir('code', [
       d.file('a.dart', _SOURCE),
     ]).create();
 
     d.dir('target_dir', [
       d.file('b.dart', _SOURCE),
     ]).create();
 
     schedule(() {
@@ skipping 78 matching lines @@
         var dir = new Directory(p.join(d.defaultRoot, 'code'));
         processDirectory(dir, overwrite: true, followLinks: true);
       }, 'running formatter');
 
       d.dir('code', [
         d.file('linked_file.dart', _FORMATTED),
       ]).validate();
     });
   }
 }