Provide a better type for line-splitter's bind.

sdk/lib/convert/line_splitter.dart

Index: sdk/lib/convert/line_splitter.dart
diff --git a/sdk/lib/convert/line_splitter.dart b/sdk/lib/convert/line_splitter.dart
index 0af2f2f63dd59c0a01b438c1008a9a60dcdb5e82..3ae958d8602c0974d320d0cbbf80a8c2a3a50d69 100644
--- a/sdk/lib/convert/line_splitter.dart
+++ b/sdk/lib/convert/line_splitter.dart
@@ -57,6 +57,8 @@ class LineSplitter extends Converter<String, List<String>> {
     }
     return new _LineSplitterSink(sink);
   }
+
+  Stream<String> bind(Stream<String> stream) => super.bind(stream);
 }
 
 // TODO(floitsch): deal with utf8.