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

tests/standalone/io/process_stderr_test.dart

Index: tests/standalone/io/process_stderr_test.dart
diff --git a/tests/standalone/io/process_stderr_test.dart b/tests/standalone/io/process_stderr_test.dart
index 6e68b9b3a5b6f40928c85e3e282c889a13cac3b1..8292b999b2356c81e08f1bbbc5711db9dfef9b9f 100644
--- a/tests/standalone/io/process_stderr_test.dart
+++ b/tests/standalone/io/process_stderr_test.dart
@@ -47,7 +47,7 @@ void test(Future<Process> future, int expectedExitCode) {
     }
 
     process.stdout.listen((_) {});
-    process.stdin.writeBytes(data);
+    process.stdin.add(data);
     process.stdin.close();
     process.stderr.listen(readData);
   });