types: Express Outbox as PmOutbox | StreamOutbox.

Like we did with `PmMessage` and `StreamMessage` in zulip#4506.

We don't actually make `PmOutbox` any different from `StreamOutbox`
in this commit; that'll come later.

src/types.js

@@ -3,7 +3,15 @@ import type { IntlShape } from 'react-intl';
 import type { DangerouslyImpreciseStyleProp } from 'react-native/Libraries/StyleSheet/StyleSheet';
 
 import type { SubsetProperties } from './generics';
-import type { Auth, Topic, Message, ReactionType, UserId } from './api/apiTypes';
+import type {
+  Auth,
+  Topic,
+  Message,
+  PmMessage,
+  StreamMessage,
+  ReactionType,
+  UserId,
+} from './api/apiTypes';
 import type { ZulipVersion } from './utils/zulipVersion';
 import type { PmKeyUsers } from './utils/recipient';
 
@@ -140,28 +148,7 @@ export type TopicExtended = {|
   unreadCount: number,
 |};
 
-/**
- * A message we're in the process of sending.
- *
- * We use these objects for two purposes:
- *
- * (a) They make up the queue of messages the user has asked us to send, and
- *     which we'll retry sending if initial attempts fail.  See
- *     `trySendMessages`.
- *
- * (b) We show them immediately in the message list, even before we've
- *     successfully gotten them to the server (but with an activity
- *     indicator to show we're still working on them.)
- *
- * Even after (a) is complete for a given message, we still need the
- * `Outbox` object for the sake of (b), until we hear an `EVENT_NEW_MESSAGE`
- * event from the server that lets us replace it with the corresponding
- * `Message` object.
- *
- * This type most often appears in the union `Message | Outbox`, and so its
- * properties are deliberately similar to those of `Message`.
- */
-export type Outbox = $ReadOnly<{|
+export type OutboxBase = $ReadOnly<{|
   /** Used for distinguishing from a `Message` object. */
   isOutbox: true,
 
@@ -180,6 +167,8 @@ export type Outbox = $ReadOnly<{|
 
   /* eslint-disable flowtype/generic-spacing */
   ...SubsetProperties<
+    // Could use `MessageBase` here, but Flow would complain anyway if
+    // we tried to put something that's not in `MessageBase` below.
     Message,
     {|
       avatar_url: mixed,
@@ -197,6 +186,41 @@ export type Outbox = $ReadOnly<{|
   >,
 |}>;
 
+export type PmOutbox = {|
+  ...OutboxBase,
+
+  ...SubsetProperties<PmMessage, {||}>,
+|};
+
+export type StreamOutbox = {|
+  ...OutboxBase,
+
+  ...SubsetProperties<StreamMessage, {||}>,
+|};
+
+/**
+ * A message we're in the process of sending.
+ *
+ * We use these objects for two purposes:
+ *
+ * (a) They make up the queue of messages the user has asked us to send, and
+ *     which we'll retry sending if initial attempts fail.  See
+ *     `trySendMessages`.
+ *
+ * (b) We show them immediately in the message list, even before we've
+ *     successfully gotten them to the server (but with an activity
+ *     indicator to show we're still working on them.)
+ *
+ * Even after (a) is complete for a given message, we still need the
+ * `Outbox` object for the sake of (b), until we hear an `EVENT_NEW_MESSAGE`
+ * event from the server that lets us replace it with the corresponding
+ * `Message` object.
+ *
+ * This type most often appears in the union `Message | Outbox`, and so its
+ * properties are deliberately similar to those of `Message`.
+ */
+export type Outbox = PmOutbox | StreamOutbox;
+
 /**
  * MessageLike: Imprecise alternative to `Message | Outbox`.
  *