Use a synchronous StreamController for daemon logs (#4028)

The logging from build_runner when running builds through a
daemon was asynchronous and was causing some `webdev build`
commands to missing the final status message.

Fixes: dart-lang/webdev#2489

Author: nshahan

build_runner/lib/src/daemon/daemon_builder.dart

@@ -231,7 +231,7 @@ class BuildRunnerDaemonBuilder implements DaemonBuilder {
     DaemonOptions daemonOptions,
   ) async {
     var expectedDeletes = <AssetId>{};
-    var outputStreamController = StreamController<ServerLog>();
+    var outputStreamController = StreamController<ServerLog>(sync: true);
 
     var environment = BuildEnvironment(
       packageGraph,