Fix all strong-mode errors and warnings.

lib/src/runner/reporter/json.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:async';
 import 'dart:convert';
 
 import '../../backend/group.dart';
 import '../../backend/live_test.dart';
 import '../../backend/metadata.dart';
@@ skipping 166 matching lines @@
     });
     return id;
   }
 
   /// Returns a list of the IDs for all the groups in [groups], which are
   /// contained in the suite identified by [suiteID].
   ///
   /// If a group doesn't have an ID yet, this assigns one and emits a new event
   /// for that group.
   List<int> _idsForGroups(Iterable<Group> groups, int suiteID) {
-    var parentID;
+    int parentID;
     return groups.map((group) {
       if (_groupIDs.containsKey(group)) {
         parentID = _groupIDs[group];
         return parentID;
       }
 
       var id = _nextID++;
       _groupIDs[group] = id;
 
       _emit("group", {
@@ skipping 45 matching lines @@
     _emit("done", {"success": success});
   }
 
   /// Emits an event with the given type and attributes.
   void _emit(String type, Map attributes) {
     attributes["type"] = type;
     attributes["time"] = _stopwatch.elapsed.inMilliseconds;
     print(JSON.encode(attributes));
   }
 }