Make pub strong-mode clean.

lib/src/command/serve.dart

 // Copyright (c) 2013, 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:math' as math;
 
 import 'package:barback/barback.dart';
 
 import '../barback/asset_environment.dart';
@@ skipping 68 matching lines @@
   }
 
   Future onRunTransformerCommand() async {
     var port = parseInt(argResults['port'], 'port');
     var adminPort = argResults['admin-port'] == null ? null :
         parseInt(argResults['admin-port'], 'admin port');
 
     var watcherType = argResults['force-poll'] ?
         WatcherType.POLLING : WatcherType.AUTO;
 
-    var environmentConstants = new Map.fromIterable(argResults["define"],
+    var environmentConstants = new Map<String, String>.fromIterable(
+        argResults["define"],
         key: (pair) => pair.split("=").first,
         value: (pair) => pair.split("=").last);
 
     var environment = await AssetEnvironment.create(entrypoint, mode,
         watcherType: watcherType, hostname: hostname, basePort: port,
         useDart2JS: useDart2JS, environmentConstants: environmentConstants);
     var directoryLength = sourceDirectories.map((dir) => dir.length)
         .reduce(math.max);
 
     if (adminPort != null) {
@@ skipping 78 matching lines @@
         "${padRight(server.rootDirectory, directoryLength)} "
         "on ${log.bold('http://$hostname:${server.port}')}");
   }
 
   /// Reports [error] and exits the server.
   void _fatalError(error, [stackTrace]) {
     if (_completer.isCompleted) return;
     _completer.completeError(error, stackTrace);
   }
 }