Make the test async so as to wait for write to be done (#58)

diff --git a/test/common_tests.dart b/test/common_tests.dart
index a8082de..89907f9 100644
--- a/test/common_tests.dart
+++ b/test/common_tests.dart
@@ -2188,7 +2188,7 @@
             isSinkClosed = true;
           });
 
-          test('throwsIfEncodingIsNullAndWriteObject', () {
+          test('throwsIfEncodingIsNullAndWriteObject', () async {
             sink.encoding = null;
             expect(() => sink.write('Hello world'), throwsNoSuchMethodError);
           });