fix strong mode messages in pkg/dev_compiler

pkg/dev_compiler/web/main.dart

Index: pkg/dev_compiler/web/main.dart
diff --git a/pkg/dev_compiler/web/main.dart b/pkg/dev_compiler/web/main.dart
index 2216580b3d5ef07decfc60a61385c52d37125fbd..a604412628da261d70ec71606cb5b9baf2158582 100755
--- a/pkg/dev_compiler/web/main.dart
+++ b/pkg/dev_compiler/web/main.dart
@@ -29,7 +29,7 @@ Future<int> _runCommand(List<String> args,
     // 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);
   }