Make Stream.fromIterable only accept a single listen call.

sdk/lib/async/stream_impl.dart

Index: sdk/lib/async/stream_impl.dart
diff --git a/sdk/lib/async/stream_impl.dart b/sdk/lib/async/stream_impl.dart
index bf7c8f015c9a51ddf3ba7a3f3a4775fb85237adc..e7cceadf7b0940ace0eb539bb6f4eb679a0c4e84 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -488,6 +488,7 @@ typedef _PendingEvents _EventGenerator();
 /** Stream that generates its own events. */
 class _GeneratedStreamImpl<T> extends _StreamImpl<T> {
   final _EventGenerator _pending;
+  bool _isUsed = false;
   /**
    * Initializes the stream to have only the events provided by a
    * [_PendingEvents].
@@ -497,6 +498,8 @@ class _GeneratedStreamImpl<T> extends _StreamImpl<T> {
   _GeneratedStreamImpl(this._pending);
 
   StreamSubscription _createSubscription(bool cancelOnError) {
+    if (_isUsed) throw new StateError("Stream has already been listened to.");
+    _isUsed = true;
     _BufferingStreamSubscription<T> subscription =
          new _BufferingStreamSubscription(cancelOnError);
     subscription._setPendingEvents(_pending());