Add folding support for plugins and update the main tutorial page

pkg/analyzer_plugin/doc/tutorial/folding.md

Index: pkg/analyzer_plugin/doc/tutorial/folding.md
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+# Providing Folding Information
+
+Folding information is used by clients to allow users to collapse portions of
+the code that are not interesting for their current task.
+
+## Implementation details
+
+Folding information is available through a subscription. If the server has
+subscribed for folding information in some set of files, then the plugin
+should send the information in an `analysis.folding` notification whenever
+the information needs to be updated.
+
+When a notification needs to be sent, the method `sendFoldingNotification`
+will be invoked. This method is responsible for sending the notification.
+
+The easiest way to add support for this notification is by adding the classes
+`FoldingMixin` and `DartFoldingMixin` (from
+`package:analyzer_plugin/plugin/folding_mixin.dart`) to the list of mixins
+for your subclass of `ServerPlugin`. This will leave you with one abstract
+method that you need to implement: `getFoldingContributors`. That method is
+responsible for returning a list of `FoldingContributor`s. It is the folding
+contributors that produce the actual folding regions. (Most plugins will only
+need a single folding contributor.)
+
+To write a folding contributor, create a class that implements
+`FoldingContributor`. The interface defines a single method named
+`computeFolding`. The method has two arguments: an `FoldingRequest` that
+describes the file for which folding information is being requested and an
+`FoldingCollector` through which folding regions are to be added.
+
+If you mix in the class `DartFoldingMixin`, then the request will be an
+instance of `DartFoldingRequest`, which also has analysis results.
+
+## Example
+
+Start by creating a class that implements `FoldingContributor`, then implement
+the method `computeFolding`. This method is typically implemented by creating a
+visitor (such as an AstVisitor) that can visit the results of the analysis (such
+as a CompilationUnit) and extract the folding regions from the analysis result.
+
+For example, your contributor might look something like the following:
+
+```dart
+class MyFoldingContributor implements FoldingContributor {
+  @override
+  void computeFolding(
+      FoldingRequest request, FoldingCollector collector) {
+    if (request is DartFoldingRequest) {
+      FoldingVisitor visitor = new FoldingVisitor(collector);
+      request.result.unit.accept(visitor);
+    }
+  }
+}
+
+class FoldingVisitor extends RecursiveAstVisitor {
+  final FoldingCollector collector;
+
+  FoldingVisitor(this.collector);
+
+  @override
+  void visitClassDeclaration(ClassDeclaration node) {
+    // ...
+  }
+}
+```
+
+Given a contributor like the one above, you can implement your plugin similar to
+the following:
+
+```dart
+class MyPlugin extends ServerPlugin with FoldingMixin, DartFoldingMixin {
+  // ...
+
+  @override
+  List<FoldingContributor> getFoldingContributors(String path) {
+    return <FoldingContributor>[new MyFoldingContributor()];
+  }
+}
+```