fix strong mode messages in pkg/dev_compiler

pkg/dev_compiler/web/main.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, 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.
 
 @JS()
 library dev_compiler.web.main;
 
 import 'dart:async';
 
@@ skipping 11 matching lines @@
 }
 
 /// Runs a single compile command, and returns an exit code.
 Future<int> _runCommand(List<String> args,
     {MessageHandler messageHandler}) async {
   try {
     // TODO: Remove CommandRunner and args if possible. May run into issues
     // with ArgResults or ArgParsers.
     var runner = new CommandRunner('dartdevc', 'Dart Development Compiler');
     runner.addCommand(new WebCompileCommand(messageHandler: messageHandler));
-    setUpCompilerInBrowser = allowInterop(await runner.run(args));
+    setUpCompilerInBrowser = allowInterop((await runner.run(args)) as Function);
   } catch (e, s) {
     return _handleError(e, s, args, messageHandler: messageHandler);
   }
   return 1;
 }
 
 /// Handles [error] in a uniform fashion. Returns the proper exit code and calls
 /// [messageHandler] with messages.
 int _handleError(dynamic error, dynamic stackTrace, List<String> args,
     {MessageHandler messageHandler}) {
@@ skipping 26 matching lines @@
     messageHandler("");
     messageHandler("    dartdevc arguments: " + args.join(' '));
     messageHandler("");
     messageHandler("```");
     messageHandler(error);
     messageHandler(stackTrace);
     messageHandler("```");
     return 70;
   }
 }