Fix all strong-mode errors and warnings.

lib/src/backend/metadata.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.
 
 import 'dart:collection';
 
 import 'package:boolean_selector/boolean_selector.dart';
 import 'package:collection/collection.dart';
 
 import '../frontend/skip.dart';
@@ skipping 40 matching lines @@
   ///
   /// This is guaranteed not to have any keys that match [tags]; those are
   /// resolved when the metadata is constructed.
   final Map<BooleanSelector, Metadata> forTag;
 
   /// Parses a user-provided map into the value for [onPlatform].
   static Map<PlatformSelector, Metadata> _parseOnPlatform(
       Map<String, dynamic> onPlatform) {
     if (onPlatform == null) return {};
 
-    var result = {};
+    var result = <PlatformSelector, Metadata>{};
     onPlatform.forEach((platform, metadata) {
       if (metadata is Timeout || metadata is Skip) {
         metadata = [metadata];
       } else if (metadata is! List) {
         throw new ArgumentError('Metadata for platform "$platform" must be a '
             'Timeout, Skip, or List of those; was "$metadata".');
       }
 
       var selector = new PlatformSelector.parse(platform);
 
@@ skipping 244 matching lines @@
   _serializeTimeout(Timeout timeout) {
     if (timeout == Timeout.none) return 'none';
     return {
       'duration': timeout.duration == null
           ? null
           : timeout.duration.inMicroseconds,
       'scaleFactor': timeout.scaleFactor
     };
   }
 }