Add a mixin for assists similar to that for fixes

pkg/analyzer_plugin/lib/utilities/assist/assist_contributor_mixin.dart

Index: pkg/analyzer_plugin/lib/utilities/assist/assist_contributor_mixin.dart
diff --git a/pkg/analyzer_plugin/lib/utilities/assist/assist_contributor_mixin.dart b/pkg/analyzer_plugin/lib/utilities/assist/assist_contributor_mixin.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c66c051d4fb666ecf556a1434ebc664d1e1c7ee9
--- /dev/null
+++ b/pkg/analyzer_plugin/lib/utilities/assist/assist_contributor_mixin.dart
@@ -0,0 +1,40 @@
+// 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/src/generated/java_core.dart';
+import 'package:analyzer_plugin/protocol/protocol_common.dart';
+import 'package:analyzer_plugin/protocol/protocol_generated.dart';
+import 'package:analyzer_plugin/utilities/assist/assist.dart';
+import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
+
+/**
+ * A partial implementation of an [AssistContributor] that provides a utility
+ * method to make it easier to add assists.
+ *
+ * Clients may not extend or implement this class, but are allowed to use it as
+ * a mix-in when creating a subclass of [AssistContributor].
+ */
+abstract class AssistContributorMixin implements AssistContributor {
+  /**
+   * The collector to which assists should be added.
+   */
+  AssistCollector get collector;
+
+  /**
+   * Add an assist. Use the [kind] of the assist to get the message and priority,
+   * and use the change [builder] to get the edits that comprise the assist. If
+   * the message has parameters, then use the list of [args] to populate the
+   * message.
+   */
+  void addAssistFromBuilder(ChangeBuilder builder, AssistKind kind,
+      {List args: null}) {

[scheglov, 2017/06/28 18:36:33]

`null` is the default default value.

[Brian Wilkerson, 2017/06/28 18:43:49]

Removed

+    SourceChange change = builder.sourceChange;
+    if (change.edits.isEmpty) {
+      return;
+    }
+    change.message = formatList(kind.message, args);
+    collector.addAssist(
+        new PrioritizedSourceChange(kind.priority, builder.sourceChange));
+  }
+}