Rework plugin API to work better with strong mode

lib/plugin.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library plugin;
 
+import 'package:plugin/src/plugin_impl.dart';
+
 /**
  * A function used to register the given [extension] to the extension point with
  * the given unique [identifier].
  *
  * An [ExtensionError] will be thrown if the [extension] is not appropriate
  * for the extension point, such as an [extension] that does not implement the
  * required interface.
  */
 typedef void RegisterExtension(String identifier, Object extension);
 
 /**
- * A function used to register an extension point with the given simple
- * [identifier]. If given, the [validator] will be used to validate extensions
- * to the extension point.
+ * A function used to register the given [extensionPoint].
  *
  * An [ExtensionError] will be thrown if the extension point cannot be
  * registered, such as when a plugin attempts to define two extension points
- * with the same simple identifier.
+ * with the same identifier.
  */
-typedef ExtensionPoint RegisterExtensionPoint(String identifier,
-    [ValidateExtension validateExtension]);
+typedef void RegisterExtensionPoint(ExtensionPoint extensionPoint);
 
 /**
  * A function used by a plugin to validate an [extension] to a extension point.
  *
  * An [ExtensionError] should be thrown if the [extension] is not valid for the
  * extension point, such as an [extension] that does not implement the required
  * interface.
  */
 typedef void ValidateExtension(Object extension);
 
@@ skipping 13 matching lines @@
    * Initialize a newly created error to have the given message.
    */
   ExtensionError(this.message);
 }
 
 /**
  * A representation of an extension point.
  *
  * Clients may not extend, implement or mix-in this class.
  */
-abstract class ExtensionPoint {
+abstract class ExtensionPoint<E> {
+  /**
+   * Initialize a newly created extension point to be defined by the given
+   * [plugin] with the given [simpleIdentifier]. The [validateExtension]
+   * function will be used to validate extensions for this extension point.
+   */
+  factory ExtensionPoint(Plugin plugin, String simpleIdentifier,
+      ValidateExtension validateExtension) = ExtensionPointImpl<E>;
+
   /**
    * Return an immutable list containing all of the extensions that were
    * registered for this extension point.
    */
-  List<Object> get extensions;
+  List<E> get extensions;
 
   /**
    * Return the plugin that defined this extension point.
    */
   Plugin get plugin;
 
   /**
    * Return the identifier used to uniquely identify this extension point within
    * the defining plugin.
    */
@@ skipping 44 matching lines @@
   static String buildUniqueIdentifier(Plugin plugin, String simpleIdentifier) =>
       join(plugin.uniqueIdentifier, simpleIdentifier);
 
   /**
    * Return an identifier created by joining the [pluginIdentifier] and the
    * [simpleIdentifier].
    */
   static String join(String pluginIdentifier, String simpleIdentifier) =>
       '$pluginIdentifier.$simpleIdentifier';
 }