Make Stream.fromIterable only accept a single listen call.

tests/lib/async/stream_from_iterable_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test merging streams.
 library dart.test.stream_from_iterable;
 
 import "package:expect/expect.dart";
 import "dart:async";
 import '../../../pkg/unittest/lib/unittest.dart';
@@ skipping 55 matching lines @@
       Duration duration = const Duration(milliseconds: 10);
       if (value == 20) {
         subscription.pause(new Future.delayed(duration, () {}));
       }
     }, onDone: expectAsync0(() {
       actual.close();
       Events expected = new Events.fromIterable(iter);
       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);
+  });
 }