Sort generated messages.

pkg/front_end/tool/_fasta/generate_messages.dart

 // Copyright (c) 2017, 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:io';
 
 import 'dart:isolate';
 
 import 'package:yaml/yaml.dart' show loadYaml;
 
@@ skipping 11 matching lines @@
 // BSD-style license that can be found in the LICENSE file.
 
 // NOTE: THIS FILE IS GENERATED. DO NOT EDIT.
 //
 // Instead modify 'pkg/front_end/messages.yaml' and run
 // 'pkg/front_end/tool/_fasta/generate_messages.dart' to update.
 
 part of fasta.codes;
 """);
 
-  yaml.forEach((String name, description) {
+  List<String> keys = yaml.keys.toList()..sort();
+  for (String name in keys) {
+    var description = yaml[name];
     while (description is String) {
       description = yaml[description];
     }
     Map map = description;
     sb.writeln(compileTemplate(name, map['template'], map['tip'],
         map['analyzerCode'], map['dart2jsCode']));
-  });
+  }
 
   String dartfmtedText = new DartFormatter().format("$sb");
 
   Uri problemsFile = await Isolate.resolvePackageUri(
       Uri.parse('package:front_end/src/fasta/fasta_codes_generated.dart'));
   await new File.fromUri(problemsFile)
       .writeAsString(dartfmtedText, flush: true);
   port.close();
 }
 
@@ skipping 122 matching lines @@
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 Message _withArguments$name(${parameters.join(', ')}) {
   ${conversions.join('\n  ')}
   return new Message(
      code$name,
      ${messageArguments.join(', ')});
 }
 """;
 }