Fix stream_controller_test failure.

sdk/lib/async/stream.dart

Index: sdk/lib/async/stream.dart
diff --git a/sdk/lib/async/stream.dart b/sdk/lib/async/stream.dart
index 5c1ff6586db7e289246bcb052c7d67b9c43cf7f7..c1699f0d636b1a22025551fcd36f9d5ccc4f3674 100644
--- a/sdk/lib/async/stream.dart
+++ b/sdk/lib/async/stream.dart
@@ -1135,6 +1135,8 @@ class _EventTransformStreamSubscription<S, T>
   final StreamEventTransformer<S, T> _transformer;
   /** Whether to unsubscribe when emitting an error. */
   final bool _unsubscribeOnError;
+  /** Whether this stream has sent a done event. */
+  bool _isClosed = false;
   /** Source of incoming events. */
   StreamSubscription<S> _subscription;
   /** Cached EventSink wrapper for this class. */
@@ -1166,9 +1168,11 @@ class _EventTransformStreamSubscription<S, T>
 
   void cancel() {
     if (_isSubscribed) {
-      _subscription.cancel();
+      StreamSubscription subscription = _subscription;
       _subscription = null;
+      subscription.cancel();
     }
+    _isClosed = true;
   }
 
   void _handleData(S data) {
@@ -1189,6 +1193,7 @@ class _EventTransformStreamSubscription<S, T>
 
   void _handleDone() {
     try {
+      _subscription = null;
       _transformer.handleDone(_sink);
     } catch (e, s) {
       _sendError(_asyncError(e, s));
@@ -1197,12 +1202,12 @@ class _EventTransformStreamSubscription<S, T>
 
   // EventOutputSink interface.
   void _sendData(T data) {
-    if (!_isSubscribed) return;
+    if (_isClosed) return;
     _onData(data);
   }
 
   void _sendError(AsyncError error) {
-    if (!_isSubscribed) return;
+    if (_isClosed) return;
     _onError(error);
     if (_unsubscribeOnError) {
       cancel();
@@ -1210,9 +1215,12 @@ class _EventTransformStreamSubscription<S, T>
   }
 
   void _sendDone() {
-    if (!_isSubscribed) return;
-    _subscription.cancel();
-    _subscription = null;
+    if (_isClosed) throw new StateError("Already closed.");
+    _isClosed = true;
+    if (_isSubscribed) {
+      _subscription.cancel();
+      _subscription = null;
+    }
     _onDone();
   }
 }