Fix most strong mode warnings.

lib/src/stream_sink_transformer/handler_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';
 
 import '../stream_sink_transformer.dart';
 import '../delegate/stream_sink.dart';
 
 /// The type of the callback for handling data events.
@@ skipping 36 matching lines @@
   final StreamSink<T> _safeCloseInner;
 
   Future get done => _inner.done;
 
   _HandlerSink(this._transformer, StreamSink<T> inner)
       : _inner = inner,
         _safeCloseInner = new _SafeCloseSink<T>(inner);
 
   void add(S event) {
     if (_transformer._handleData == null) {
-      // [event] is an S and [_inner.add] takes a T. This style of conversion
-      // will throw an error in checked mode if [_inner] is actually a
-      // [StreamSink<T>], but will work if [_inner] isn't reified and won't add
-      // an extra check in unchecked mode.
-      _inner.add(event as dynamic);
+      _inner.add(event as T);
     } else {
       _transformer._handleData(event, _safeCloseInner);
     }
   }
 
   void addError(error, [StackTrace stackTrace]) {
     if (_transformer._handleError == null) {
       _inner.addError(error, stackTrace);
     } else {
       _transformer._handleError(error, stackTrace, _safeCloseInner);
@@ skipping 24 matching lines @@
 class _SafeCloseSink<T> extends DelegatingStreamSink<T> {
   _SafeCloseSink(StreamSink<T> inner) : super(inner);
 
   Future close() => super.close().catchError((_) {});
 }
 
 /// A function to pass as a [StreamTransformer]'s `handleDone` callback.
 void _closeSink(EventSink sink) {
   sink.close();
 }