Fix some strong mode issues in the core libraries.

sdk/lib/convert/converter.dart

Index: sdk/lib/convert/converter.dart
diff --git a/sdk/lib/convert/converter.dart b/sdk/lib/convert/converter.dart
index 5c380cd5b78533432a5fbfb1eddb01481ba72431..711c3fe151c10a4f6d3571956f37b46125138616 100644
--- a/sdk/lib/convert/converter.dart
+++ b/sdk/lib/convert/converter.dart
@@ -50,8 +50,7 @@ abstract class Converter<S, T> implements StreamTransformer<S, T> {
  *
  * For a non-chunked conversion converts the input in sequence.
  */
-class _FusedConverter<S, M, T> extends Converter<S, T>
-    implements ChunkedConverter<S, T, S, T> {
+class _FusedConverter<S, M, T> extends Converter<S, T> {
   final Converter<S, M> _first;
   final Converter<M, T> _second;