Minor enhancement to the plugin tutorials

pkg/analyzer_plugin/doc/tutorial/fixes.md

Index: pkg/analyzer_plugin/doc/tutorial/fixes.md
diff --git a/pkg/analyzer_plugin/doc/tutorial/fixes.md b/pkg/analyzer_plugin/doc/tutorial/fixes.md
index 3135c83c93aee1e1d30c3d39188f60f8e1041064..c81548c46c1550ed284b3b24072e9077ef58c78b 100644
--- a/pkg/analyzer_plugin/doc/tutorial/fixes.md
+++ b/pkg/analyzer_plugin/doc/tutorial/fixes.md
@@ -48,9 +48,12 @@ single fix contributor.)
 To write a fix contributor, create a class that implements `FixContributor`. The
 interface defines a single method named `computeFixes`. The method has two
 arguments: a `FixesRequest` that describes the errors that should be fixed and a
-`FixCollector` through which fixes are to be added. (If you use the mixins above
-then the list of errors available through the request object will only include
-the errors for which fixes should be returned.)
+`FixCollector` through which fixes are to be added.
+
+If you mix in the class `DartFixesMixin`, then the list of errors available
+through the request object will only include the errors for which fixes should
+be returned and the request will be an instance of `DartFixesRequest`, which
+also has analysis results.
 
 The class `FixContributorMixin` defines a simple implementation of this method
 that captures the two arguments in fields, iterates through the errors, and