server: create output directory if needed

lib/devc.dart

 // Copyright (c) 2015, 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.
 
 /// Command line tool to run the checker on a Dart program.
 library dev_compiler.devc;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ skipping 248 matching lines @@
     }
     var port = options.port;
     _log.fine('Serving $entryPath at http://0.0.0.0:$port/');
     var compiler = new Compiler(options);
     return new CompilerServer._(compiler, outDir, port, entryPath);
   }
 
   CompilerServer._(this.compiler, this.outDir, this.port, this._entryPath);
 
   Future start() async {
+    // Create output directory if needed. shelf_static will fail otherwise.
+    var out = new Directory(outDir);
+    if (!await out.exists()) await out.create(recursive: true);
+
     var handler = const shelf.Pipeline()
         .addMiddleware(rebuildAndCache)
         .addHandler(shelf_static.createStaticHandler(outDir,
             defaultDocument: _entryPath));
     await shelf.serve(handler, '0.0.0.0', port);
     compiler.run();
   }
 
   shelf.Handler rebuildAndCache(shelf.Handler handler) => (request) {
     _log.fine('requested $GREEN_COLOR${request.url}$NO_COLOR');
@@ skipping 19 matching lines @@
       // Note: the cache-control header should be enough, but this doesn't hurt
       // and can help renew the policy after it expires.
       headers['ETag'] = segments[1];
     }
     return response.change(headers: headers);
   };
 }
 
 final _log = new Logger('dev_compiler');
 final _earlyErrorResult = new CheckerResults(const [], null, true);