dyn keyword required on 0.2.85

[bobbobbio]

Starting with 0.2.85, in at least this one particular instance the wasm_bindgen macro seems to need the dyn keyword to compile correctly, and otherwise it gives a strange error. For specifics see this compilation error with stdweb koute/stdweb#427

I guess this is a breaking change but I'm not sure if this was intentional or not. I think the dyn keyword isn't required on all editions, but I'm not sure what editions are suppose to be supported.

If the answer is stdweb just needs to be updated, then this can be closed.

[daxpedda]

Duplicate of #3415.

The problem is syn in this case, it correctly requires to use the dyn keyword when writing trait objects like this. This happened because of the upgrade to v2 of syn, so there is nothing we, as in wasm-bindgen, can really do here. This should be fixed in stdweb.