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

test/command_line_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.command_line;
 
 import 'dart:convert';
 
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/descriptor.dart' as d;
@@ skipping 192 matching lines @@
 
     test("errors if the indent is not a non-negative number", () {
       var process = runFormatter(["--indent", "notanum"]);
       process.shouldExit(64);
 
       process = runFormatter(["--preserve", "-4"]);
       process.shouldExit(64);
     });
   });
 
+  group("--set-exit-if-changed", () {
+    test("gives exit code 0 if there are no changes", () {
+      d.dir("code", [d.file("a.dart", formattedSource)]).create();
+
+      var process = runFormatterOnDir(["--set-exit-if-changed"]);
+      process.shouldExit(0);
+    });
+
+    test("gives exit code 1 if there are changes", () {
+      d.dir("code", [d.file("a.dart", unformattedSource)]).create();
+
+      var process = runFormatterOnDir(["--set-exit-if-changed"]);
+      process.shouldExit(1);
+    });
+
+    test("gives exit code 1 if there are changes even in dry run", () {
+      d.dir("code", [d.file("a.dart", unformattedSource)]).create();
+
+      var process = runFormatterOnDir(["--set-exit-if-changed", "--dry-run"]);
+      process.shouldExit(1);
+    });
+  });
+
   group("with no paths", () {
     test("errors on --overwrite", () {
       var process = runFormatter(["--overwrite"]);
       process.shouldExit(64);
     });
 
     test("exits with 65 on parse error", () {
       var process = runFormatter();
       process.writeLine("herp derp i are a dart");
       process.closeStdin();
       process.shouldExit(65);
     });
 
     test("reads from stdin", () {
       var process = runFormatter();
       process.writeLine(unformattedSource);
       process.closeStdin();
 
       // No trailing newline at the end.
       process.stdout.expect(formattedSource.trimRight());
       process.shouldExit(0);
     });
   });
 }