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

lib/src/io.dart

Index: lib/src/io.dart
diff --git a/lib/src/io.dart b/lib/src/io.dart
index 579ed1b2db25286d7d042c1034dfc2ee1671ea7d..e1910332a83d7354ec8cceceac4ce814491fd592 100644
--- a/lib/src/io.dart
+++ b/lib/src/io.dart
@@ -38,10 +38,15 @@ void processFile(File file, {String label, bool overwrite, int pageWidth}) {
 
   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);