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

tests/standalone/io/process_broken_pipe_test.dart

Index: tests/standalone/io/process_broken_pipe_test.dart
diff --git a/tests/standalone/io/process_broken_pipe_test.dart b/tests/standalone/io/process_broken_pipe_test.dart
index 275bce104c05e2f8ddc06d04a4d36e5ffcff57d8..01451bf5bba39f66e275ba46102a75b9b101309e 100644
--- a/tests/standalone/io/process_broken_pipe_test.dart
+++ b/tests/standalone/io/process_broken_pipe_test.dart
@@ -25,7 +25,7 @@ main() {
     // Write to the stdin after the process is terminated to test
     // writing to a broken pipe.
     process.exitCode.then((code) {
-      process.stdin.writeBytes([0]);
+      process.stdin.add([0]);
     });
   });
 }