modelTypes: Move .stream_id from Message to StreamMessage.

Flow gives us one substantive bit of feedback and one kind of false
alarm:

- We shouldn't be putting a `stream_id` on PMs in exampleData; so,
  stop doing that.

- When we start putting things together to call
  `addItemsToStreamArray`, we need to be sure `action.message` is a
  stream message. We were already sure of that; there's already an
  early return on `action.message.type !== stream`. But that early
  return needs to be just a bit later in order to satisfy Flow that
  `action.message` can't be a stream message. In particular, as Greg
  explains [1],

  """
  Specifically what's happening is: we check `action.message.type`,
  but then we call `getOwnUserId` -- and that's some function that
  might go and do anything. So in particular it might go and mutate
  the same object that we're looking at as `action.message`. So that
  object *had* had a `type` of `'private'`, but now it might not and
  so it might not have a `stream_id`.
  """

[1] zulip#4506 (comment)

 In particular, Flow
  sees the `getOwnUserId` call, and that's some function that might
  go and do anything -- including

But Flow seems
  to think it's done too early, and that something in the
  intervening code might thwart the job it's doing. So, move that
  early return a bit later.

Author: chrisbobbe

src/__tests__/lib/exampleData.js

@@ -348,7 +348,6 @@ export const pmMessage = (args?: {|
     // in real messages.  (See comments on the Message type.)
     display_recipient: recipients.map(displayRecipientFromUser),
     id: randMessageId(),
-    stream_id: -1,
     subject: '',
     timestamp: 1556579160,
     type: 'private',

src/api/modelTypes.js

@@ -543,8 +543,6 @@ type MessageBase = $ReadOnly<{|
    */
   display_recipient: string | $ReadOnlyArray<PmRecipientUser>, // `string` for type stream, else PmRecipientUser[]
 
-  stream_id: number, // FixMe: actually only for type `stream`, else absent.
-
   subject: string,
   subject_links: $ReadOnlyArray<string>,
 |}>;
@@ -561,6 +559,8 @@ export type StreamMessage = $ReadOnly<{|
 
   // TODO: Put stream-message fields here.
   type: 'stream',
+
+  stream_id: number,
 |}>;
 
 /**

src/unread/unreadStreamsReducer.js

@@ -16,11 +16,11 @@ import { getOwnUserId } from '../users/userSelectors';
 const initialState: UnreadStreamsState = NULL_ARRAY;
 
 const eventNewMessage = (state, action, globalState) => {
-  if (action.message.type !== 'stream') {
+  if (getOwnUserId(globalState) === action.message.sender_id) {
     return state;
   }
 
-  if (getOwnUserId(globalState) === action.message.sender_id) {
+  if (action.message.type !== 'stream') {
     return state;
   }