msglist diffing tests [nfc]: Comment about msgWithChoice's .submessages

For where we got the 1.9 threshold, see
  https://chat.zulip.org/#narrow/stream/412-api-documentation/topic/.60.2Esubmessages.60.20on.20message.20objects/near/1358489

Author: chrisbobbe

src/api/modelTypes.js

@@ -623,7 +623,8 @@ type MessageBase = $ReadOnly<{|
   sender_realm_str: string,
   sender_short_name: string,
 
-  /** Older servers omit this; when omitted, equivalent to empty array. */
+  /** Servers <1.9.0 omit this; when omitted, equivalent to empty array. */
+  // TODO(server-1.9): Make required.
   submessages?: $ReadOnlyArray<Submessage>,
 
   timestamp: number,

src/webview/__tests__/generateInboundEventEditSequence-test.js

@@ -842,6 +842,10 @@ describe('getEditSequence correct for interesting changes', () => {
       const msgWithVote = {
         ...baseMessage,
         submessages: [
+          // Flow v0.149 will invalidate a refinement that `.submessages` is
+          // present. We can assert it's present with `invariant` in an
+          // IIFE, or use a $FlowFixMe
+          // TODO(server-1.9): No special treatment needed.
           ...msgWithChoice.submessages,
           {
             ...baseSubmessage,