Add utilities to support the outline notification

pkg/analyzer_plugin/doc/tutorial/outline.md

+# Providing Outlines
+
+Outline information is used by clients to allow users to see the structure of
+their code.
+
+## Implementation details
+
+Outline information can is available through a subscription. If the server has

[scheglov, 2017/08/24 18:29:20]

"can" or "is"?

[Brian Wilkerson, 2017/08/24 18:52:46]

Done ('is')

+subscribed for outline information in some set of files, the the plugin should

[scheglov, 2017/08/24 18:29:20]

"the the" => "then the"?

[Brian Wilkerson, 2017/08/24 18:52:46]

Done

+send the information in an `analysis.outline` notification whenever the
+information needs to be updated.
+
+When a notification needs to be sent, the method`sendOutlineNotification` 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
+`OutlineMixin` and `DartOutlineMixin` (from
+`package:analyzer_plugin/plugin/outline_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: `getOutlineContributors`. That method is
+responsible for returning a list of `OutlineContributor`s. It is the outline
+contributors that produce the actual outline information. (Most plugins will
+only need a single outline contributor.)
+
+To write an outline contributor, create a class that implements
+`OutlineContributor`. The interface defines a single method named
+`computeOutline`. The method has two arguments: an `OutlineRequest` that
+describes the file for which outline information is being requested and an
+`OutlineCollector` through which outline information is to be added.
+
+If you mix in the class `DartOutlineMixin`, then the request will be an instance
+of `DartOutlineRequest`, which also has analysis results.
+
+## Example
+
+Start by creating a class that implements `OutlineContributor`, then
+implement the method `computeOutline`. 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 outline information from
+the analysis result.
+
+For example, your contributor might look something like the following:
+
+```dart
+class MyOutlineContributor implements OutlineContributor {
+  @override
+  void computeOutline(
+      OutlineRequest request, OutlineCollector collector) {
+    if (request is DartOutlineRequest) {
+      OutlineVisitor visitor = new OutlineVisitor(collector);
+      request.result.unit.accept(visitor);
+    }
+  }
+}
+
+class OutlineVisitor extends RecursiveAstVisitor {
+  final OutlineCollector collector;
+
+  OutlineVisitor(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 OutlineMixin, DartOutlineMixin {
+  // ...
+
+  @override
+  List<OutlineContributor> getOutlineContributors(String path) {
+    return <OutlineContributor>[new MyOutlineContributor()];
+  }
+}
+```