Fix most strong mode warnings.

lib/src/single_subscription_transformer.dart

Index: lib/src/single_subscription_transformer.dart
diff --git a/lib/src/single_subscription_transformer.dart b/lib/src/single_subscription_transformer.dart
index 4255175f5d797a68111b5c3f978d515ce821c263..e01efac0041095e0d5d31e746bb1a3ef9bbfbfca 100644
--- a/lib/src/single_subscription_transformer.dart
+++ b/lib/src/single_subscription_transformer.dart
@@ -9,15 +9,26 @@ import 'dart:async';
 ///
 /// 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;
   }
 }