DAS plugins: Add docs for writing a plugin.

Here we provide docs specifically for writing a plugin, using a plugin, and we
improve the text about writing rules.

Change-Id: I3fcfbc50609054158e5bdf964499005a79b4fba8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410160
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server_plugin/doc/using_plugins.md

@@ -0,0 +1,39 @@
+# Using plugins
+
+This document describes how to enable an analyzer plugin. An analyzer plugin
+can be enabled for a given package, so that the plugin can report diagnostics
+(lints and warnings) and offer quick fixes. Plugins are enabled via the
+`analysis_options.yaml` file under the top-level `plugins` section:
+
+```
+plugins:
+  my_plugin: ^1.0.0
+```
+
+Note: This is similar to how analyzer plugins are enabled in the [legacy][]
+analyzer plugin system. However, in the legacy system, this `plugins` section
+is listed under the top-level `analyzer` section. In the new analyzer plugin
+system, `plugins` is a top-level section.
+
+Individual plugins are listed similar to how dependencies are listed in a
+`pubspec.yaml` file; they are listed as a key-value pair, with the package name
+as the key. The value can either be
+
+* a package version constraint, in which case the package is downloaded from
+  https://pub.dev,
+* a git dependency,
+* an absolute path.
+
+For example, while developing a plugin locally, it can be enabled as:
+
+```
+plugins:
+  my_plugin:
+    path: /path/to/my_plugin
+```
+
+Note: after any change is made to the `plugins` section of an
+`analysis_options.yaml` file, the Dart Analysis Server must be restarted to see
+the effects.
+
+[legacy]: https://github.com/dart-lang/sdk/blob/main/pkg/analyzer_plugin/doc/tutorial/tutorial.md

pkg/analysis_server_plugin/doc/writing_a_plugin.md

@@ -0,0 +1,67 @@
+# Writing a plugin
+
+This document describes how to write an analyzer plugin, to provide custom
+static analysis, or to offer custom quick fixes in an IDE.
+
+## The pubspec file
+
+An analyzer plugin is a Dart package, so let's start with the `pubspec.yaml` file:
+
+```yaml
+name: test_analyzer_plugin
+version: 0.0.1
+
+environment:
+  sdk: '>=3.6.0 <4.0.0'
+
+dependencies:
+  analysis_server_plugin: any
+  analyzer: ^7.2.0
+```
+
+There is nothing special about this pubspec; note that we need a dependency on
+the `analysis_server_plugin` package, and on the `analyzer` package, supporting
+at least 7.2.0. The version of the analyzer package needs to move lockstep with
+the Dart SDK. For Dart 3.8.0-70.1.beta, `^7.2.0` is a good version constraint.
+
+## The main Dart file
+
+One source file is required, at `lib/main.dart`. Here is the basic layout:
+
+```dart
+import 'package:analysis_server_plugin/plugin.dart';
+import 'package:analysis_server_plugin/registry.dart';
+
+final plugin = SimplePlugin();
+
+class SimplePlugin extends Plugin {
+  @override
+  void register(PluginRegistry registry) {
+    // Here we register analysis rules, and quick fixes.
+  }
+}
+```
+
+Here we have a class, `SimplePlugin`, which extends the `Plugin` class from the
+`analysis_server_plugin` package. This class has one method that we override:
+`register`. In the `register` method, we can register analysis rules and quick
+fixes (CorrectionProducers). See details in the [writing rules][] doc, and the
+writing quick fixes doc (TODO).
+
+Additionally, we provide a top-level variable in this file called `plugin`,
+which is an instance of our `SimplePlugin` class. When a running instance of
+the Dart Analysis Server needs to use this analyzer plugin, it generates some
+code that needs to _import_ this `lib/main.dart` file, and that references this
+`plugin` top-level variable.
+
+# Debugging a plugin
+
+If a plugin is not behaving as expected (for example, warnings are not
+appearing in the IDE), you can check the [analyzer diagnostics pages][]. If the
+plugin isolate has crashed, the "plugins" screen will display the crash.
+
+`print` cannot be used in plugin code to debug. Instead, writing to a log file
+can help in debugging plugin code.
+
+[writing rules]: https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server_plugin/doc/writing_rules.md
+[analyzer diagnostics pages]: https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server/doc/tutorial/instrumentation.md#open-the-analyzer-diagnostics-pages

pkg/analysis_server_plugin/doc/writing_rules.md

@@ -26,7 +26,7 @@ class MyRule extends AnalysisRule {
 
   MyRule()
       : super(
-          name: LintNames.prefer_void_to_null,
+          name: 'my_rule',
           description: 'A longer description of the rule.',
         );
 
@@ -124,6 +124,13 @@ Let's look at each declaration individually:
   analysis. Typically, a 'visit' method like this is where we perform some
   analysis and maybe report lint(s) or warning(s).
 
+Some rules do not require complex logic in the visitor class, but rules may
+also need to walk up or down the syntax tree, or examine properties of nodes
+carefully and thoroughly. For many examples of analysis rules and their visitor
+classes, see the [lint rules] that ship with the Dart Analysis Server.
+
+[lint rules]: https://github.com/dart-lang/sdk/tree/main/pkg/linter/lib/src/rules
+
 ## Registering an analysis rule
 
 In order for an analysis rule to be used in an analyzer plugin, it must be
@@ -144,4 +151,6 @@ enabled by default. To register an analysis rule as a "lint rule," such that it
 must be specifically enabled from analysis options, use `registerLintRule`
 instead.
 
-TODO(srawlins): Write up and link documentation for this Plugin subclass.
+See [writing a plugin][] for for information about the `Plugin` class.
+
+[writing a plugin]: https://github.com/dart-lang/sdk/blob/main/pkg/analysis_server_plugin/doc/writing_rules.md