Annotate MyDecoder.add in web example to satisfy type_annotate_public_apis

dart analyze flagged the public override `add(obj)` in the web example for
a missing type annotation. Annotate it as `dynamic add(dynamic obj)` to
match the base Decoder.add signature, leaving `dart analyze` clean.

Author: jumperchen

web/main.dart

@@ -50,7 +50,7 @@ class MyEncoder extends Encoder {
 
 class MyDecoder extends Decoder {
   @override
-  add(obj) {
+  dynamic add(dynamic obj) {
     print('MyDecoder: $obj');
     return super.add(obj);
   }