Create a public API for contributing assists and make assists pluggable

pkg/analysis_server/lib/plugin/assist.dart

Index: pkg/analysis_server/lib/plugin/assist.dart
diff --git a/pkg/analysis_server/lib/plugin/fix.dart b/pkg/analysis_server/lib/plugin/assist.dart
similarity index 60%
copy from pkg/analysis_server/lib/plugin/fix.dart
copy to pkg/analysis_server/lib/plugin/assist.dart
index a89b84738cb5957ee1afd190df31dd8553c88791..cc037f6b087576f8b012c42a6b767dd37be074f3 100644
--- a/pkg/analysis_server/lib/plugin/fix.dart
+++ b/pkg/analysis_server/lib/plugin/assist.dart
@@ -3,20 +3,20 @@
 // BSD-style license that can be found in the LICENSE file.
 
 /**
- * Support for client code that extends the analysis server by adding new fix
+ * Support for client code that extends the analysis server by adding new assist
  * contributors.
  */
-library analysis_server.plugin.fix;
+library analysis_server.plugin.assist;
 
-import 'package:analysis_server/edit/fix/fix_core.dart';
+import 'package:analysis_server/edit/assist/assist_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].
+ * assist contributors with the server. The object used as an extension must be
+ * an [AssistContributor].
  */
-final String FIX_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
+final String ASSIST_CONTRIBUTOR_EXTENSION_POINT_ID = Plugin.join(
     ServerPlugin.UNIQUE_IDENTIFIER,
-    ServerPlugin.FIX_CONTRIBUTOR_EXTENSION_POINT);
+    ServerPlugin.ASSIST_CONTRIBUTOR_EXTENSION_POINT);