Do not require a return value from cross-frame methods (RPC) about client HOT 1 CLOSED

Ah! The situation we have here is a "notification" (see 4.1 on https://www.jsonrpc.org/specification). I didn't realize the spec made the distinction. This is effectively one-way communication from the client-sidebar frame to the LMS frame. The proper way to sort this is to:

• extend the sidebar's postmessage-json-rpc with a notify function or similar. Its behavior would be subset of call. It would not set an id on the request object sent via postMessage. It would then send the message to the frame via postMessage but not expect a response (i.e. not set up listeners).
• On the LMS side, update postmessage-json-rpc#_jsonRPCResponse to not send a response to "notifications" (requests with no id property).

The LMS-side change would need to be made first such that it wouldn't throw an error if a request object does not have an id property.

from client.