Split out error codes into separate files

pkg/analyzer/lib/src/dart/error/lint_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.lint_codes;
+
+import 'package:analyzer/error/error.dart';
+
+/**
+ * Defines style and best practice recommendations.
+ *
+ * Unlike [HintCode]s, which are akin to traditional static warnings from a
+ * compiler, lint recommendations focus on matters of style and practices that
+ * might aggregated to define a project's style guide.
+ */
+class LintCode extends ErrorCode {
+  const LintCode(String name, String message, [String correction])
+      : super(name, message, correction);
+
+  @override
+  ErrorSeverity get errorSeverity => ErrorSeverity.INFO;
+
+  @override
+  ErrorType get type => ErrorType.LINT;
+}