narrow [nfc]: Explicitly convert to ApiNarrow where appropriate.

This conversion is a no-op.  But we'll shortly be converting `Narrow`
into an opaque type -- which will make it a no-op that only the
`narrow` module can perform.  That means Flow will check for us that
we've explicitly marked all the places where we intend this
conversion to happen; these are those places.

src/message/fetchActions.js

@@ -24,7 +24,7 @@ import {
   MESSAGE_FETCH_COMPLETE,
 } from '../actionConstants';
 import { FIRST_UNREAD_ANCHOR, LAST_MESSAGE_ANCHOR } from '../anchor';
-import { ALL_PRIVATE_NARROW } from '../utils/narrow';
+import { ALL_PRIVATE_NARROW, apiNarrowOfNarrow } from '../utils/narrow';
 import { BackoffMachine } from '../utils/async';
 import { initNotifications } from '../notification/notificationActions';
 import { addToOutbox, sendOutbox } from '../outbox/outboxActions';
@@ -88,6 +88,7 @@ export const fetchMessages = (fetchArgs: {|
   try {
     const { messages, found_newest, found_oldest } = await api.getMessages(getAuth(getState()), {
       ...fetchArgs,
+      narrow: apiNarrowOfNarrow(fetchArgs.narrow),
       useFirstUnread: fetchArgs.anchor === FIRST_UNREAD_ANCHOR, // TODO: don't use this; see #4203
     });
     dispatch(
@@ -235,7 +236,7 @@ export const fetchMessagesInNarrow = (
 const fetchPrivateMessages = () => async (dispatch: Dispatch, getState: GetState) => {
   const auth = getAuth(getState());
   const { messages, found_newest, found_oldest } = await api.getMessages(auth, {
-    narrow: ALL_PRIVATE_NARROW,
+    narrow: apiNarrowOfNarrow(ALL_PRIVATE_NARROW),
     anchor: LAST_MESSAGE_ANCHOR,
     numBefore: 100,
     numAfter: 0,

src/utils/narrow.js

@@ -29,7 +29,7 @@ import {
  *    into a data structure;
  *  * `caseNarrow` and its relatives, for pattern-matching or destructuring;
  *  * `ApiNarrow` for the form we put a narrow in when talking to the
- *    server.
+ *    server, and `apiNarrowOfNarrow` for converting to it.
  */
 export type Narrow = ApiNarrow;
 
@@ -459,6 +459,11 @@ export const isStreamOrTopicNarrow = (narrow?: Narrow): boolean =>
 export const isSearchNarrow = (narrow?: Narrow): boolean =>
   !!narrow && caseNarrowDefault(narrow, { search: () => true }, () => false);
 
+/**
+ * Convert the narrow into the form used in the Zulip API at get-messages.
+ */
+export const apiNarrowOfNarrow = (narrow: Narrow): ApiNarrow => narrow;
+
 /**
  * True just if the given message is part of the given narrow.
  *