Make EventTransformerStream's subscription not send events after cancel.

tests/lib/async/first_regression_test.dart

+// Copyright (c) 2012, 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.
+
+// Regression test for http://dartbug.com/7964
+
+library first_regression_test;
+import 'dart:async';
+import '../../../pkg/unittest/lib/unittest.dart';
+
+class DoubleTransformer<T> extends StreamEventTransformer<T, T> {
+  void handleData(T data, EventSink<T> sink) {
+    sink.add(data);
+    sink.add(data);
+  }
+}
+
+main() {
+  test("Double event before first", () {
+    // This should not crash. Did crash by trying to complete future more
+    // than once.
+    new Stream.fromIterable([1, 2])
+        .transform(new DoubleTransformer())
+        .first
+        .then(expectAsync1((e) {}));
+  });
+}