Rework plugin API to work better with strong mode

lib/src/plugin_impl.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.src.plugin_impl;
 
 import 'dart:collection';
 
 import 'package:plugin/manager.dart';
 import 'package:plugin/plugin.dart';
 
 /**
  * A concrete implementation of an [ExtensionManager].
  */
 class ExtensionManagerImpl implements ExtensionManager {
   /**
    * A table mapping the id's of extension points to the corresponding
    * extension points.
    */
   Map<String, ExtensionPointImpl> extensionPoints =
       new HashMap<String, ExtensionPointImpl>();
 
   @override
   void processPlugins(List<Plugin> plugins) {
     for (Plugin plugin in plugins) {
-      plugin.registerExtensionPoints((String identifier,
-              [ValidateExtension validateExtension]) =>
-          registerExtensionPoint(plugin, identifier, validateExtension));
+      plugin.registerExtensionPoints(registerExtensionPoint);
     }
     for (Plugin plugin in plugins) {
       plugin.registerExtensions(registerExtension);
     }
   }
 
   /**
    * Register an [extension] to the extension point with the given unique
    * [identifier].
    */
   void registerExtension(String identifier, Object extension) {
     ExtensionPointImpl extensionPoint = extensionPoints[identifier];
     if (extensionPoint == null) {
       throw new ExtensionError(
           'There is no extension point with the id "$identifier"');
     }
     extensionPoint.add(extension);
   }
 
   /**
-   * Register an extension point being defined by the given [plugin] with the
-   * given simple [identifier] and [validateExtension].
+   * Register the given [extensionPoint].
    */
-  ExtensionPoint registerExtensionPoint(
-      Plugin plugin, String identifier, ValidateExtension validateExtension) {
-    String uniqueIdentifier = Plugin.buildUniqueIdentifier(plugin, identifier);
+  void registerExtensionPoint(ExtensionPoint extensionPoint) {
+    String uniqueIdentifier = extensionPoint.uniqueIdentifier;
     if (extensionPoints.containsKey(uniqueIdentifier)) {
       throw new ExtensionError(
-          'There is already an extension point with the id "$identifier"');
+          'There is already an extension point with the id "$uniqueIdentifier"');
     }
-    ExtensionPointImpl extensionPoint =
-        new ExtensionPointImpl(plugin, identifier, validateExtension);
-    extensionPoints[uniqueIdentifier] = extensionPoint;
-    return extensionPoint;
+    extensionPoints[uniqueIdentifier] = extensionPoint as ExtensionPointImpl;
   }
 }
 
 /**
  * A concrete representation of an extension point.
  */
-class ExtensionPointImpl implements ExtensionPoint {
+class ExtensionPointImpl<E> implements ExtensionPoint<E> {
   @override
   final Plugin plugin;
 
   @override
   final String simpleIdentifier;
 
   /**
    * The function used to validate extensions to this extension point.
    */
   final ValidateExtension validateExtension;
 
   /**
    * The list of extensions to this extension point.
    */
-  final List<Object> _extensions = <Object>[];
+  final List<E> _extensions = <E>[];
 
   /**
    * Initialize a newly create extension point to belong to the given [plugin]
    * and have the given [simpleIdentifier]. If [validateExtension] is non-`null`
    * it will be used to validate extensions associated with this extension
    * point.
    */
   ExtensionPointImpl(
       this.plugin, this.simpleIdentifier, this.validateExtension);
 
   /**
    * Return a list containing all of the extensions that have been registered
    * for this extension point.
    */
-  List<Object> get extensions => new UnmodifiableListView(_extensions);
+  List<E> get extensions => new UnmodifiableListView<E>(_extensions);
 
   /**
    * Return the identifier used to uniquely identify this extension point. The
    * unique identifier is the identifier for the plugin, followed by a period
    * (`.`), followed by the [simpleIdentifier] for the extension point.
    */
   String get uniqueIdentifier =>
       Plugin.buildUniqueIdentifier(plugin, simpleIdentifier);
 
   /**
    * Validate that the given [extension] is appropriate for this extension
    * point, and if it is then add it to the list of registered exceptions.
    */
   void add(Object extension) {
     if (validateExtension != null) {
       validateExtension(extension);
     }
-    _extensions.add(extension);
+    _extensions.add(extension as E);
   }
 }