Add generic argument to Codec.fuse.

Fixes #27722
BUG= http://dartbug.com/27722
R=lrn@google.com

Review URL: https://codereview.chromium.org/2472023004 .

sdk/lib/convert/codec.dart

@@ -62,8 +62,8 @@ abstract class Codec<S, T> {
    */
   // TODO(floitsch): use better example with line-splitter once that one is
   // in this library.
-  Codec<S, dynamic> fuse(Codec<T, dynamic> other) {
-    return new _FusedCodec<S, T, dynamic>(this, other);
+  Codec<S, dynamic/*=R*/> fuse/*<R>*/(Codec<T, dynamic/*=R*/> other) {
+    return new _FusedCodec<S, T, dynamic/*=R*/>(this, other);
   }
 
   /**