server: create output directory if needed

lib/devc.dart

Index: lib/devc.dart
diff --git a/lib/devc.dart b/lib/devc.dart
index bfe3d5d43ff02e5e4ff391c2e6fcdf95886344a8..980e2db1393619b81606b616217139632f34e3b9 100644
--- a/lib/devc.dart
+++ b/lib/devc.dart
@@ -266,6 +266,10 @@ class CompilerServer {
   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,