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

bin/format.dart

Index: bin/format.dart
diff --git a/bin/format.dart b/bin/format.dart
index 72182bf0ea96f98cc9f08d597d3bbb754ede9e60..a8a00e2480abcade376ba9ea308100350f48ee96 100644
--- a/bin/format.dart
+++ b/bin/format.dart
@@ -33,6 +33,9 @@ void main(List<String> args) {
       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,
@@ -119,6 +122,10 @@ void main(List<String> args) {
     reporter = new ProfileReporter(reporter);
   }
 
+  if (argResults["set-exit-if-changed"]) {
+    reporter = new SetExitReporter(reporter);
+  }
+
   var pageWidth;
   try {
     pageWidth = int.parse(argResults["line-length"]);