Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/third_party/plugin/lib/manager.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.manager;
+
+import 'package:plugin/src/plugin_impl.dart';
+import 'package:plugin/plugin.dart';
+
+/**
+ * An object that manages the extension points for the host application.
+ */
+abstract class ExtensionManager {
+  /**
+   * Initialize a newly created manager.
+   */
+  factory ExtensionManager() = ExtensionManagerImpl;
+
+  /**
+   * Process each of the given [plugins] by allowing them to register extension
+   * points and extensions.
+   *
+   * An [ExtensionError] will be thrown if any of the plugins throws such an
+   * exception while registering with this manager.
+   */
+  void processPlugins(List<Plugin> plugins);
+}