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..d6a065503bf586db2bf84ed3e018c5500b058464 100644
--- a/sdk/lib/async/stream_transformers.dart
+++ b/sdk/lib/async/stream_transformers.dart
@@ -209,12 +209,19 @@ class _HandlerEventSink<S, T> implements EventSink<S> {
   final _TransformDoneHandler<T> _handleDone;
 
   /// The output sink where the handlers should send their data into.
-  final EventSink<T> _sink;
+  EventSink<T> _sink;
 
   _HandlerEventSink(
-      this._handleData, this._handleError, this._handleDone, this._sink);
+      this._handleData, this._handleError, this._handleDone, this._sink) {
+    if (_sink == null) {
+      throw new ArgumentError("The provided sink must not be null.");
+    }
+  }
+
+  bool get _isClosed => _sink == null;
 
   void add(S data) {
+    if (_isClosed) throw new StateError("Sink is closed");
     if (_handleData != null) {
       _handleData(data, _sink);
     } else {
@@ -223,6 +230,7 @@ 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 {
@@ -231,10 +239,13 @@ class _HandlerEventSink<S, T> implements EventSink<S> {
   }
 
   void close() {
+    if (_isClosed) return;
+    var sink = _sink;
+    _sink = null;
     if (_handleDone != null) {
-      _handleDone(_sink);
+      _handleDone(sink);
     } else {
-      _sink.close();
+      sink.close();
     }
   }
 }