Add a command-line option to set the exit code on a formatting change.

bin/format.dart

 // Copyright (c) 2015, 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';
 
 import 'package:dart_style/src/dart_formatter.dart';
@@ skipping 15 matching lines @@
   parser.addOption("line-length",
       abbr: "l", help: "Wrap lines longer than this.", defaultsTo: "80");
   parser.addOption("indent",
       abbr: "i", help: "Spaces of leading indentation.", defaultsTo: "0");
   parser.addOption("preserve",
       help: 'Selection to preserve, formatted as "start:length".');
   parser.addFlag("dry-run",
       abbr: "n",
       negatable: false,
       help: "Show which files would be modified but make no changes.");
+  parser.addFlag("set-exit-if-changed",
+      negatable: false,
+      help: "Return exit code 1 if there are any formatting changes.");
   parser.addFlag("overwrite",
       abbr: "w",
       negatable: false,
       help: "Overwrite input files with formatted output.");
   parser.addFlag("machine",
       abbr: "m",
       negatable: false,
       help: "Produce machine-readable JSON output.");
   parser.addFlag("profile",
       negatable: false, help: "Display profile times after running.");
@@ skipping 66 matching lines @@
 
     reporter = OutputReporter.overwrite;
   } else if (argResults["machine"]) {
     reporter = OutputReporter.printJson;
   }
 
   if (argResults["profile"]) {
     reporter = new ProfileReporter(reporter);
   }
 
+  if (argResults["set-exit-if-changed"]) {
+    reporter = new SetExitReporter(reporter);
+  }
+
   var pageWidth;
   try {
     pageWidth = int.parse(argResults["line-length"]);
   } on FormatException catch (_) {
     usageError(
         parser,
         '--line-length must be an integer, was '
         '"${argResults['line-length']}".');
   }
 
@@ skipping 111 matching lines @@
     output = stdout;
   }
 
   output.write("""$message
 
 Usage: dartfmt [-n|-w] [files or directories...]
 
 ${parser.usage}
 """);
 }