Make new StreamController be async by default.

sdk/lib/_internal/pub/lib/src/io.dart

Index: sdk/lib/_internal/pub/lib/src/io.dart
diff --git a/sdk/lib/_internal/pub/lib/src/io.dart b/sdk/lib/_internal/pub/lib/src/io.dart
index e9a13167539da2e8b797058d4c407224af1170b9..e9d803b3e2911bac1c09b3cfe2f85237a05ce53a 100644
--- a/sdk/lib/_internal/pub/lib/src/io.dart
+++ b/sdk/lib/_internal/pub/lib/src/io.dart
@@ -422,7 +422,7 @@ Future drainStream(Stream stream) {
 /// will succeed when [EventSink] is closed or fail with any errors that occur
 /// while writing.
 Pair<EventSink, Future> consumerToSink(StreamConsumer consumer) {
-  var controller = new StreamController();
+  var controller = new StreamController(sync: true);
   var done = controller.stream.pipe(consumer);
   return new Pair<EventSink, Future>(controller.sink, done);
 }
@@ -739,7 +739,7 @@ ByteStream createTarGz(List contents, {baseDir}) {
   contents.forEach((file) => buffer.write('$file\n'));
   log.fine(buffer.toString());
 
-  var controller = new StreamController<List<int>>();
+  var controller = new StreamController<List<int>>(sync: true);
 
   if (baseDir == null) baseDir = path.current;
   baseDir = path.absolute(baseDir);