Option to enable showOffsets in dump.dart.

pkg/kernel/bin/dump.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, 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 'package:kernel/kernel.dart';
 
 main(args) {
   var binary = loadProgramFromBinary(args[0]);
-  writeProgramToText(binary, path: args[1]);
+  writeProgramToText(binary,
+      path: args[1], showOffsets: const bool.fromEnvironment("showOffsets"));
 }