Mark plugin as strong mode compliant

.analysis_options

diff --git a/.analysis_options b/.analysis_options
new file mode 100644
index 0000000..a10d4c5
--- /dev/null
+++ b/.analysis_options
@@ -0,0 +1,2 @@
+analyzer:
+  strong-mode: true

lib/plugin.dart

diff --git a/lib/plugin.dart b/lib/plugin.dart
index b0176f8..66cebb0 100644
--- a/lib/plugin.dart
+++ b/lib/plugin.dart
@@ -39,7 +39,7 @@
  * An exception indicating that an error occurred while attempting to register
  * either an extension or an extension point.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 class ExtensionError implements Exception {
   /**
@@ -56,7 +56,7 @@
 /**
  * A representation of an extension point.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 abstract class ExtensionPoint {
   /**
@@ -88,7 +88,7 @@
  * A contribution to the host application that can extend the behavior of the
  * application while also allowing other plugins to extend it's behavior.
  *
- * Clients are expected to subtype this class when implementing plugins.
+ * Clients may implement this class when implementing plugins.
  */
 abstract class Plugin {
   /**