Warn when adding something to a closed sink and improve documentation

sdk/lib/async/stream_transformers.dart

Index: sdk/lib/async/stream_transformers.dart
diff --git a/sdk/lib/async/stream_transformers.dart b/sdk/lib/async/stream_transformers.dart
index bfc83d6b58b4befbe215bd641956492567b85380..ff2983ddae0cb11b665225b7c0dd6a2bf145b4e7 100644
--- a/sdk/lib/async/stream_transformers.dart
+++ b/sdk/lib/async/stream_transformers.dart
@@ -210,11 +210,13 @@ class _HandlerEventSink<S, T> implements EventSink<S> {
 
   /// The output sink where the handlers should send their data into.
   final EventSink<T> _sink;

[Lasse Reichstein Nielsen, 2017/04/26 08:26:19]

Make this field non-final and set it to null when you close.
Then change _isClosed to 
  bool get _isClosed => _sink == null;

That saves one field.

[floitsch, 2017/05/01 16:46:25]

Done.

+  bool _isClosed = false;
 
   _HandlerEventSink(
       this._handleData, this._handleError, this._handleDone, this._sink);
 
   void add(S data) {
+    if (_isClosed) throw new StateError("Sink is closed");
     if (_handleData != null) {
       _handleData(data, _sink);
     } else {
@@ -223,6 +225,7 @@ class _HandlerEventSink<S, T> implements EventSink<S> {
   }
 
   void addError(Object error, [StackTrace stackTrace]) {
+    if (_isClosed) throw new StateError("Sink is closed");
     if (_handleError != null) {
       _handleError(error, stackTrace, _sink);
     } else {
@@ -231,6 +234,7 @@ class _HandlerEventSink<S, T> implements EventSink<S> {
   }
 
   void close() {

[Lasse Reichstein Nielsen, 2017/04/26 08:26:19]

We should probably bail out here if the sink is already closed.
So:
  if (_isClosed) return;

[floitsch, 2017/05/01 16:46:25]

Realized the same thing.
done.

+    _isClosed = true;
     if (_handleDone != null) {
       _handleDone(_sink);
     } else {