Remove StreamSink(replaced by EventSink) and make IOSink extend EventSink.

tests/standalone/io/process_stdout_test.dart

Index: tests/standalone/io/process_stdout_test.dart
diff --git a/tests/standalone/io/process_stdout_test.dart b/tests/standalone/io/process_stdout_test.dart
index 2188f947cc8c802809da16f34822d5b138e7f341..0bce7aa2e80c8919b6e38d44f9d9c51e3677b8c4 100644
--- a/tests/standalone/io/process_stdout_test.dart
+++ b/tests/standalone/io/process_stdout_test.dart
@@ -47,7 +47,7 @@ void test(Future<Process> future, int expectedExitCode) {
     }
 
     process.stderr.listen((_) {});
-    process.stdin.writeBytes(data);
+    process.stdin.add(data);
     process.stdin.close();
     process.stdout.listen(readData);
   });