Fix most strong mode warnings.

lib/src/single_subscription_transformer.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 
 /// A transformer that converts a broadcast stream into a single-subscription
 /// stream.
 ///
 /// This buffers the broadcast stream's events, which means that it starts
 /// listening to a stream as soon as it's bound.
+///
+/// This also casts the source stream's events to type `T`. If the cast fails,
+/// the result stream will emit a [CastError]. This behavior is deprecated, and
+/// should not be relied upon.
 class SingleSubscriptionTransformer<S, T> implements StreamTransformer<S, T> {
   const SingleSubscriptionTransformer();
 
   Stream<T> bind(Stream<S> stream) {
     var subscription;
-    var controller = new StreamController(sync: true,
+    var controller = new StreamController<T>(sync: true,
         onCancel: () => subscription.cancel());
-    subscription = stream.listen(controller.add,
-        onError: controller.addError, onDone: controller.close);
+    subscription = stream.listen((value) {
+      // TODO(nweiz): When we release a new major version, get rid of the second
+      // type parameter and avoid this conversion.
+      try {
+        controller.add(value as T);
+      } on CastError catch (error, stackTrace) {
+        controller.addError(error, stackTrace);
+      }
+    }, onError: controller.addError, onDone: controller.close);
     return controller.stream;
   }
 }