Rework plugin API to work better with strong mode

test/plugin_impl_test.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 test.operation;
 
 import 'package:plugin/plugin.dart';
 import 'package:plugin/src/plugin_impl.dart';
 import 'package:unittest/unittest.dart';
 
 main() {
   groupSep = ' | ';
 
   group('ExtensionManager', () {
     test('processPlugins', () {
       TestPlugin plugin1 = new TestPlugin('plugin1');
       TestPlugin plugin2 = new TestPlugin('plugin1');
       ExtensionManagerImpl manager = new ExtensionManagerImpl();
       manager.processPlugins([plugin1, plugin2]);
       expect(plugin1.extensionPointsRegistered, true);
       expect(plugin1.extensionsRegistered, true);
       expect(plugin2.extensionPointsRegistered, true);
       expect(plugin2.extensionsRegistered, true);
     });
 
     test('registerExtension - valid', () {
       Plugin plugin = new TestPlugin('plugin');
       ExtensionManagerImpl manager = new ExtensionManagerImpl();
-      ExtensionPoint point =
-          manager.registerExtensionPoint(plugin, 'point', null);
+      ExtensionPoint point = new ExtensionPoint(plugin, 'point', null);
+      manager.registerExtensionPoint(point);
       expect(point, isNotNull);
       Object extension = 'extension';
       manager.registerExtension('plugin.point', extension);
       List<Object> extensions = point.extensions;
       expect(extensions, isNotNull);
       expect(extensions, hasLength(1));
       expect(extensions[0], extension);
     });
 
     test('registerExtension - non existent', () {
       ExtensionManagerImpl manager = new ExtensionManagerImpl();
       expect(() => manager.registerExtension('does not exist', 'extension'),
           throwsA(new isInstanceOf<ExtensionError>()));
       ;
     });
 
     test('registerExtensionPoint - non-conflicting', () {
       Plugin plugin1 = new TestPlugin('plugin1');
       Plugin plugin2 = new TestPlugin('plugin2');
       ExtensionManagerImpl manager = new ExtensionManagerImpl();
-      expect(
-          manager.registerExtensionPoint(plugin1, 'point1', null), isNotNull);
-      expect(
-          manager.registerExtensionPoint(plugin1, 'point2', null), isNotNull);
-      expect(
-          manager.registerExtensionPoint(plugin2, 'point1', null), isNotNull);
-      expect(
-          manager.registerExtensionPoint(plugin2, 'point2', null), isNotNull);
+      manager
+          .registerExtensionPoint(new ExtensionPoint(plugin1, 'point1', null));
+      manager
+          .registerExtensionPoint(new ExtensionPoint(plugin1, 'point2', null));
+      manager
+          .registerExtensionPoint(new ExtensionPoint(plugin2, 'point1', null));
+      manager
+          .registerExtensionPoint(new ExtensionPoint(plugin2, 'point2', null));
     });
 
     test('registerExtensionPoint - conflicting - same plugin', () {
       Plugin plugin1 = new TestPlugin('plugin1');
       ExtensionManagerImpl manager = new ExtensionManagerImpl();
+      manager
+          .registerExtensionPoint(new ExtensionPoint(plugin1, 'point1', null));
       expect(
-          manager.registerExtensionPoint(plugin1, 'point1', null), isNotNull);
-      expect(() => manager.registerExtensionPoint(plugin1, 'point1', null),
+          () => manager.registerExtensionPoint(
+              new ExtensionPoint(plugin1, 'point1', null)),
           throwsA(new isInstanceOf<ExtensionError>()));
     });
 
     test('registerExtensionPoint - conflicting - different plugins', () {
       Plugin plugin1 = new TestPlugin('plugin1');
       Plugin plugin2 = new TestPlugin('plugin1');
       ExtensionManagerImpl manager = new ExtensionManagerImpl();
+      manager
+          .registerExtensionPoint(new ExtensionPoint(plugin1, 'point1', null));
       expect(
-          manager.registerExtensionPoint(plugin1, 'point1', null), isNotNull);
-      expect(() => manager.registerExtensionPoint(plugin2, 'point1', null),
+          () => manager.registerExtensionPoint(
+              new ExtensionPoint(plugin2, 'point1', null)),
           throwsA(new isInstanceOf<ExtensionError>()));
     });
   });
 
   group('ExtensionPointImpl', () {
     test('extensions - empty', () {
       Plugin plugin = new TestPlugin('plugin');
       ExtensionPointImpl point = new ExtensionPointImpl(plugin, 'point', null);
       List<Object> extensions = point.extensions;
       expect(extensions, isNotNull);
@@ skipping 23 matching lines @@
       point.add('extension 1');
       point.add('extension 2');
       point.add('extension 3');
       List<Object> extensions = point.extensions;
       expect(extensions, isNotNull);
       expect(extensions, hasLength(3));
     });
 
     test('add - with validator - valid', () {
       Plugin plugin = new TestPlugin('plugin');
-      ExtensionPointImpl point = new ExtensionPointImpl(plugin, 'point',
-          (Object extension) {
+      ExtensionPointImpl point =
+          new ExtensionPointImpl(plugin, 'point', (Object extension) {
         if (extension is! String) {
           throw new ExtensionError('');
         }
       });
       point.add('extension');
     });
 
     test('add - with validator - invalid', () {
       Plugin plugin = new TestPlugin('plugin');
-      ExtensionPointImpl point = new ExtensionPointImpl(plugin, 'point',
-          (Object extension) {
+      ExtensionPointImpl point =
+          new ExtensionPointImpl(plugin, 'point', (Object extension) {
         if (extension is! String) {
           throw new ExtensionError('');
         }
       });
       expect(() => point.add(1), throwsA(new isInstanceOf<ExtensionError>()));
     });
   });
 }
 
 /**
@@ skipping 22 matching lines @@
   @override
   void registerExtensionPoints(RegisterExtensionPoint register) {
     extensionPointsRegistered = true;
   }
 
   @override
   void registerExtensions(RegisterExtension register) {
     extensionsRegistered = true;
   }
 }