Start making server strong mode clean

pkg/analysis_server/lib/src/provisional/completion/dart/completion_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 analysis_server.src.provisional.completion.dart.plugin;
 
 import 'package:analysis_server/src/provisional/completion/completion.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion.dart';
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/services/completion/dart/arglist_contributor.dart';
@@ skipping 37 matching lines @@
   /**
    * The extension point that allows plugins to register Dart specific
    * completion contributor factories.
    */
   ExtensionPoint _contributorExtensionPoint;
 
   /**
    * Return a list containing all of the Dart specific completion contributors.
    */
   Iterable<DartCompletionContributor> get contributors =>
-      _contributorExtensionPoint.extensions
-          .map((DartCompletionContributorFactory factory) => factory());
+      _contributorExtensionPoint.extensions.map(
+          (Object factory) => (factory as DartCompletionContributorFactory)());
 
   @override
   String get uniqueIdentifier => UNIQUE_IDENTIFIER;
 
   @override
   void registerExtensionPoints(RegisterExtensionPoint registerExtensionPoint) {
     _contributorExtensionPoint = registerExtensionPoint(
         CONTRIBUTOR_EXTENSION_POINT,
         _validateDartCompletionContributorExtension);
   }
@@ skipping 52 matching lines @@
    * valid Dart specific completion contributor.
    */
   void _validateDartCompletionContributorExtension(Object extension) {
     if (extension is! DartCompletionContributorFactory) {
       String id = _contributorExtensionPoint.uniqueIdentifier;
       throw new ExtensionError(
           'Extensions to $id must be a DartCompletionContributorFactory');
     }
   }
 }