Add a mixin for assists similar to that for fixes

pkg/analyzer_plugin/lib/utilities/assist/assist.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 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer_plugin/protocol/protocol.dart';
 import 'package:analyzer_plugin/protocol/protocol_generated.dart';
 import 'package:analyzer_plugin/src/utilities/assist/assist.dart';
 import 'package:analyzer_plugin/utilities/generator.dart';
@@ skipping 57 matching lines @@
                 false, exception.toString(), stackTrace.toString())
             .toNotification());
       }
     }
     EditGetAssistsResult result = new EditGetAssistsResult(collector.assists);
     return new GeneratorResult(result, notifications);
   }
 }
 
 /**
+ * A description of a class of assists. Instances are intended to hold the
+ * information that is common across a number of assists and to be shared by
+ * those assists.
+ *
+ * Clients may not extend, implement or mix-in this class.
+ */
+class AssistKind {
+  /**
+   * The name of this kind of assist, used for debugging.
+   */
+  final String name;
+
+  /**
+   * The priority of this kind of assist for the kind of error being addressed.
+   */
+  final int priority;
+
+  /**
+   * A human-readable description of the changes that will be applied by this
+   * kind of assist. The message can contain parameters, where each parameter is
+   * represented by a zero-based index inside curly braces. For example, the
+   * message `"Create a component named '{0}' in '{1}'"` contains two parameters.
+   */
+  final String message;
+
+  /**
+   * Initialize a newly created kind of assist to have the given [name],
+   * [relevance] and [message].
+   */
+  const AssistKind(this.name, this.priority, this.message);
+
+  @override
+  String toString() => name;
+}
+
+/**
  * The information about a requested set of assists.
  *
  * Clients may not extend, implement or mix-in this class.
  */
 abstract class AssistRequest {
   /**
    * Return the length of the selection within the source for which assists are
    * being requested.
    */
   int get length;
@@ skipping 15 matching lines @@
  * `.dart` file.
  *
  * Clients may not extend, implement or mix-in this class.
  */
 abstract class DartAssistRequest implements AssistRequest {
   /**
    * The analysis result for the file in which the assists are being requested.
    */
   ResolveResult get result;
 }