Create a public API for contributing fixes and make fixes pluggable

pkg/analysis_server/lib/plugin/fix.dart

+// Copyright (c) 2015, 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.
+
+/**
+ * Support for client code that extends the analysis server by adding new fix
+ * contributors.
+ */
+library analysis_server.plugin.fix;
+
+import 'package:analysis_server/edit/fix/fix_core.dart';
+import 'package:analysis_server/src/plugin/server_plugin.dart';
+import 'package:analyzer/plugin/plugin.dart';
+
+/**
+ * The identifier of the extension point that allows plugins to register new
+ * fix contributors with the server. The object used as an extension must be a
+ * [FixContributor].
+ */
+final String FIX_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
+    ServerPlugin.UNIQUE_IDENTIFIER,
+    ServerPlugin.FIX_CONTRIBUTOR_EXTENSION_POINT);