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 b6f4524238321ce322320a7756289d4c6d1cb287..67e0f9221b6af0d3ac89ccd9586ac685a670e2ea 100644
--- a/sdk/lib/io/string_transformer.dart
+++ b/sdk/lib/io/string_transformer.dart
@@ -62,9 +62,6 @@ class _WindowsCodePageEncoder
     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);
 }
 
@@ -114,9 +111,6 @@ class _WindowsCodePageDecoder
     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);
 }