Fix more strong mode errors and warnings.

lib/src/command/build.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 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
 
 import '../barback/asset_environment.dart';
@@ skipping 69 matching lines @@
         }
       });
 
       // If we're using JSON output, the regular server logging is disabled.
       // Instead, we collect it here to include in the final JSON result.
       if (log.json.enabled) {
         environment.barback.log.listen(
             (entry) => logJson.add(_logEntryToJson(entry)));
       }
 
-      var assets = await log.progress("Building ${entrypoint.root.name}",
-          () async {
+      var assets = await log.progress/*<AssetSet>*/(
+          "Building ${entrypoint.root.name}", () async {
         // Register all of the build directories.
         // TODO(rnystrom): We don't actually need to bind servers for these, we
         // just need to add them to barback's sources. Add support to
         // BuildEnvironment for going the latter without the former.
         await Future.wait(sourceDirectories.map(
             (dir) => environment.serveDirectory(dir)));
 
         return environment.barback.getAllAssets();
       });
 
@@ skipping 155 matching lines @@
         "end": {
           "line": entry.span.end.line,
           "column": entry.span.end.column
         },
       };
     }
 
     return data;
   }
 }