Add support for implementing getAssists in plugins

pkg/analyzer_plugin/lib/src/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_generated.dart';
+import 'package:analyzer_plugin/utilities/assist/assist.dart';
+
+/**
+ * A concrete implementation of [AssistCollector].
+ */
+class AssistCollectorImpl implements AssistCollector {
+  /**
+   * The list of assists that have been collected.
+   */
+  final List<PrioritizedSourceChange> assists = <PrioritizedSourceChange>[];
+
+  @override
+  void addAssist(PrioritizedSourceChange assist) {
+    assists.add(assist);
+  }
+}
+
+/**
+ * A concrete implementation of [AssistRequest].
+ */
+class AssistRequestImpl implements AssistRequest {
+  @override
+  final ResourceProvider resourceProvider;
+
+  @override
+  final int offset;
+
+  @override
+  final int length;
+
+  @override
+  final ResolveResult result;
+
+  /**
+   * Initialize a newly create request with the given data.
+   */
+  AssistRequestImpl(
+      this.resourceProvider, this.offset, this.length, this.result);
+}