Make Stream.fromIterable only accept a single listen call.

tests/lib/async/stream_from_iterable_test.dart

Index: tests/lib/async/stream_from_iterable_test.dart
diff --git a/tests/lib/async/stream_from_iterable_test.dart b/tests/lib/async/stream_from_iterable_test.dart
index ab7d543724aa7dc4ef760d599f5af415e51f4520..7398fe0e51f130151c9d27d35d20742dd2ec7a30 100644
--- a/tests/lib/async/stream_from_iterable_test.dart
+++ b/tests/lib/async/stream_from_iterable_test.dart
@@ -73,4 +73,11 @@ main() {
       Expect.listEquals(expected.events, actual.events);
     }));
   });
+
+  test("iterable-single-subscription", () {
+    Stream stream = new Stream.fromIterable(iter);
+    stream.listen((x){});
+    Expect.throws(() { stream.listen((x){}); },
+                  (e) => e is StateError);
+  });
 }