Fix strong mode errors in dart:io.

sdk/lib/io/string_transformer.dart

Index: sdk/lib/io/string_transformer.dart
diff --git a/sdk/lib/io/string_transformer.dart b/sdk/lib/io/string_transformer.dart
index 16e721b4baf05c66616629da2fea70a4e17774f2..ab17eafea121a94179b492ded02187ab2f26c3b3 100644
--- a/sdk/lib/io/string_transformer.dart
+++ b/sdk/lib/io/string_transformer.dart
@@ -61,9 +61,6 @@ class _WindowsCodePageEncoder extends Converter<String, List<int>> {
     return new _WindowsCodePageEncoderSink(sink);
   }
 
-  // Override the base-class' bind, to provide a better type.
-  Stream<List<int>> bind(Stream<String> stream) => super.bind(stream);
-
   external static List<int> _encodeString(String string);
 }
 
@@ -112,9 +109,6 @@ class _WindowsCodePageDecoder extends Converter<List<int>, String> {
     return new _WindowsCodePageDecoderSink(sink);
   }
 
-  // Override the base-class' bind, to provide a better type.
-  Stream<String> bind(Stream<List<int>> stream) => super.bind(stream);
-
   external static String _decodeBytes(List<int> bytes);
 }