Fix more strong mode errors and warnings.

lib/src/barback/web_socket_api.dart

 // Copyright (c) 2014, 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:io';
 
 import 'package:path/path.dart' as path;
 import 'package:json_rpc_2/json_rpc_2.dart' as json_rpc;
 import 'package:web_socket_channel/web_socket_channel.dart';
@@ skipping 203 matching lines @@
   ///     }
   ///
   /// If successful, it returns a map containing the URL that can be used to
   /// access the directory.
   ///
   ///     "result": {
   ///       "url": "http://localhost:8083"
   ///     }
   ///
   /// If the directory is already being served, returns the previous URL.
-  Future<Map> _serveDirectory(json_rpc.Parameters params) {
+  Future<Map> _serveDirectory(json_rpc.Parameters params) async {
     var rootDirectory = _validateRelativePath(params, "path");
-    return _environment.serveDirectory(rootDirectory).then((server) {
+    try {
+      var server = await _environment.serveDirectory(rootDirectory);
       return {
         "url": server.url.toString()
       };
-    }).catchError((error) {
-      if (error is! OverlappingSourceDirectoryException) throw error;
-
+    } on OverlappingSourceDirectoryException catch (error) {
       var dir = pluralize("directory", error.overlappingDirectories.length,
           plural: "directories");
       var overlapping = toSentence(error.overlappingDirectories.map(
           (dir) => '"$dir"'));
       print("data: ${error.overlappingDirectories}");
       throw new json_rpc.RpcException(_Error.OVERLAPPING,
           'Path "$rootDirectory" overlaps already served $dir $overlapping.',
           data: {
             "directories": error.overlappingDirectories
           });
-    });
+    }
   }
 
   /// Given a relative directory path within the entrypoint package, unbinds
   /// the server previously bound to that directory and returns its (now
   /// unreachable) URL.
   ///
   /// The method name is "unserveDirectory" and it takes a "path" key (a
   /// native OS path relative to the root of the entrypoint package) for the
   /// directory being unserved:
   ///
@@ skipping 46 matching lines @@
 }
 
 /// The pub-specific JSON RPC error codes.
 class _Error {
   /// The specified directory is not being served.
   static const NOT_SERVED = 1;
 
   /// The specified directory overlaps one or more ones already being served.
   static const OVERLAPPING = 2;
 }