Split out error codes into separate files

pkg/analyzer/lib/src/dart/error/todo_codes.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 analyzer.src.dart.error.todo_codes;
+
+import 'package:analyzer/error/error.dart';
+
+/**
+ * The error code indicating a marker in code for work that needs to be finished
+ * or revisited.
+ */
+class TodoCode extends ErrorCode {
+  /**
+   * The single enum of TodoCode.
+   */
+  static const TodoCode TODO = const TodoCode('TODO');
+
+  /**
+   * This matches the two common Dart task styles
+   *
+   * * TODO:
+   * * TODO(username):
+   *
+   * As well as
+   * * TODO
+   *
+   * But not
+   * * todo
+   * * TODOS
+   */
+  static RegExp TODO_REGEX =
+      new RegExp("([\\s/\\*])((TODO[^\\w\\d][^\\r\\n]*)|(TODO:?\$))");
+
+  /**
+   * Initialize a newly created error code to have the given [name].
+   */
+  const TodoCode(String name) : super(name, "{0}");
+
+  @override
+  ErrorSeverity get errorSeverity => ErrorSeverity.INFO;
+
+  @override
+  ErrorType get type => ErrorType.TODO;
+}