Add folding support for plugins and update the main tutorial page

pkg/analyzer_plugin/lib/plugin/folding_mixin.dart

Index: pkg/analyzer_plugin/lib/plugin/folding_mixin.dart
diff --git a/pkg/analyzer_plugin/lib/plugin/folding_mixin.dart b/pkg/analyzer_plugin/lib/plugin/folding_mixin.dart
new file mode 100644
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+++ b/pkg/analyzer_plugin/lib/plugin/folding_mixin.dart
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+// 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 'dart:async';
+
+import 'package:analyzer/dart/analysis/results.dart';
+import 'package:analyzer/src/dart/analysis/driver.dart';
+import 'package:analyzer_plugin/plugin/plugin.dart';
+import 'package:analyzer_plugin/protocol/protocol.dart';
+import 'package:analyzer_plugin/src/utilities/folding/folding.dart';
+import 'package:analyzer_plugin/utilities/folding/folding.dart';
+import 'package:analyzer_plugin/utilities/generator.dart';
+
+/**
+ * A mixin that can be used when creating a subclass of [ServerPlugin] and
+ * mixing in [FoldingMixin]. This implements the creation of the folding
+ * request based on the assumption that the driver being created is an
+ * [AnalysisDriver].
+ *
+ * Clients may not extend or implement this class, but are allowed to use it as
+ * a mix-in when creating a subclass of [ServerPlugin] that also uses
+ * [FoldingMixin] as a mix-in.
+ */
+abstract class DartFoldingMixin implements FoldingMixin {
+  @override
+  Future<FoldingRequest> getFoldingRequest(String path) async {
+    ResolveResult result = await getResolveResult(path);
+    return new DartFoldingRequestImpl(resourceProvider, result);
+  }
+}
+
+/**
+ * A mixin that can be used when creating a subclass of [ServerPlugin] to
+ * provide most of the implementation for producing folding notifications.
+ *
+ * Clients may not extend or implement this class, but are allowed to use it as
+ * a mix-in when creating a subclass of [ServerPlugin].
+ */
+abstract class FoldingMixin implements ServerPlugin {
+  /**
+   * Return a list containing the folding contributors that should be used
+   * to create folding regions for the file with the given [path].
+   */
+  List<FoldingContributor> getFoldingContributors(String path);
+
+  /**
+   * Return the folding request that should be passes to the contributors
+   * returned from [getFoldingContributors].
+   *
+   * Throw a [RequestFailure] if the request could not be created.
+   */
+  Future<FoldingRequest> getFoldingRequest(String path);
+
+  @override
+  Future<Null> sendFoldingNotification(String path) async {
+    try {
+      FoldingRequest request = await getFoldingRequest(path);
+      FoldingGenerator generator =
+          new FoldingGenerator(getFoldingContributors(path));
+      GeneratorResult generatorResult =
+          await generator.generateFoldingNotification(request);
+      generatorResult.sendNotifications(channel);
+    } on RequestFailure {
+      // If we couldn't analyze the file, then don't send a notification.
+    }
+  }
+}