Warn when adding something to a closed sink and improve documentation

sdk/lib/io/io_sink.dart

Index: sdk/lib/io/io_sink.dart
diff --git a/sdk/lib/io/io_sink.dart b/sdk/lib/io/io_sink.dart
index bae95792db6edafc5d70b7833865b6a8a0eb612b..75c8d9877e40f7cec26513d8bfb7214fc69af7a2 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -148,11 +148,12 @@ class _StreamSinkImpl<T> implements StreamSink<T> {
   _StreamSinkImpl(this._target);
 
   void add(T data) {
-    if (_isClosed) return;
+    if (_isClosed) throw new StateError("StreamSink is closed");
     _controller.add(data);
   }
 
   void addError(error, [StackTrace stackTrace]) {
+    if (_isClosed) throw new StateError("StreamSink is closed");
     _controller.addError(error, stackTrace);
   }