Drop the _WhereType transformer definition (dart-lang/stream_transform#98)

This was only required because of the challenges of being exposed as a
`StreamTransformer` and the subtyping rules for the generic arguments.
Now that we aren't exposing it as `StreamTransformer` we don't need
extra complications.

pkgs/stream_transform/lib/src/where.dart

diff --git a/pkgs/stream_transform/lib/src/where.dart b/pkgs/stream_transform/lib/src/where.dart
index 850e9a9..8917cb4 100644
--- a/pkgs/stream_transform/lib/src/where.dart
+++ b/pkgs/stream_transform/lib/src/where.dart
@@ -17,7 +17,7 @@
   /// [S] should be a subtype of the stream's generic type, otherwise nothing of
   /// type [S] could possibly be emitted, however there is no static or runtime
   /// checking that this is the case.
-  Stream<S> whereType<S>() => transform(_WhereType<S>());
+  Stream<S> whereType<S>() => where((e) => e is S).cast<S>();
 
   /// Like [where] but allows the [test] to return a [Future].
   ///
@@ -54,36 +54,3 @@
     }));
   }
 }
-
-class _WhereType<R> extends StreamTransformerBase<Null, R> {
-  @override
-  Stream<R> bind(Stream<Object> source) {
-    var controller = source.isBroadcast
-        ? StreamController<R>.broadcast(sync: true)
-        : StreamController<R>(sync: true);
-
-    StreamSubscription<Object> subscription;
-    controller.onListen = () {
-      assert(subscription == null);
-      subscription = source.listen(
-          (value) {
-            if (value is R) controller.add(value);
-          },
-          onError: controller.addError,
-          onDone: () {
-            subscription = null;
-            controller.close();
-          });
-      if (!source.isBroadcast) {
-        controller
-          ..onPause = subscription.pause
-          ..onResume = subscription.resume;
-      }
-      controller.onCancel = () {
-        subscription?.cancel();
-        subscription = null;
-      };
-    };
-    return controller.stream;
-  }
-}

pkgs/stream_transform/pubspec.yaml

diff --git a/pkgs/stream_transform/pubspec.yaml b/pkgs/stream_transform/pubspec.yaml
index 62a7fa6..9f98a71 100644
--- a/pkgs/stream_transform/pubspec.yaml
+++ b/pkgs/stream_transform/pubspec.yaml
@@ -1,7 +1,7 @@
 name: stream_transform
 description: A collection of utilities to transform and manipulate streams.
 homepage: https://www.github.com/dart-lang/stream_transform
-version: 1.1.1
+version: 1.1.2-dev
 
 environment:
   sdk: ">=2.6.0 <3.0.0"