[ffigen] Use one NativeCallable.listener for all listener blocks

[liamappelbe]

Atm we have one NativeCallable.listener per block signature. Each NativeCallable.listener has its own receive port. This means that two callbacks with different signatures can be called in a different order to the order that the native thread called them in. This can cause problems, especially for protocol implementers.

To fix this we can use a single NativeCallable.listener for all blocks in the isolate. The downside of this is it'll involve dynamic dispatch, which is slightly inefficient. But I think that's negligible in this async case.

dart-lang/http#1413

[liamappelbe]

The messages arrive in order even when the block signatures are different (#1781), so using a single listener won't help. Gonna work on this approach instead: #1647