flags reducer: Assert flags present on EVENT_NEW_MESSAGE action.

Much like we did for the various `unread` sub-reducers in the
previous commit, this simplifies a bit the potential paths through
this code.

There was a test for this impossible case; remove that.

While we're here, also remove a similar impossible check on
`messages` being falsy, as if we got an EVENT_UPDATE_MESSAGE_FLAGS
action with no `messages` property.

src/chat/__tests__/flagsReducer-test.js

@@ -76,21 +76,6 @@ describe('flagsReducer', () => {
   });
 
   describe('EVENT_NEW_MESSAGE', () => {
-    test('when no flags key is passed, do not fail, do nothing', () => {
-      const prevState = NULL_OBJECT;
-
-      const action = deepFreeze({
-        type: EVENT_NEW_MESSAGE,
-        message: { id: 2 },
-      });
-
-      const expectedState = {};
-
-      const actualState = flagsReducer(prevState, action);
-
-      expect(actualState).toEqual(expectedState);
-    });
-
     test('adds to store flags from new message', () => {
       const prevState = NULL_OBJECT;
 

src/chat/flagsReducer.js

@@ -1,4 +1,6 @@
 /* @flow strict-local */
+import invariant from 'invariant';
+
 import type { Action, FlagsState, Message } from '../types';
 import {
   REALM_INIT,
@@ -29,9 +31,9 @@ const initialState = {
 const addFlagsForMessages = (
   state: FlagsState,
   messages: $ReadOnlyArray<number>,
-  flags?: $ReadOnlyArray<string>,
+  flags: $ReadOnlyArray<string>,
 ): FlagsState => {
-  if (!messages || messages.length === 0 || !flags || flags.length === 0) {
+  if (messages.length === 0 || flags.length === 0) {
     return state;
   }
 
@@ -111,8 +113,10 @@ export default (state: FlagsState = initialState, action: Action): FlagsState =>
     case MESSAGE_FETCH_COMPLETE:
       return processFlagsForMessages(state, action.messages);
 
-    case EVENT_NEW_MESSAGE:
+    case EVENT_NEW_MESSAGE: {
+      invariant(action.message.flags, 'message in EVENT_NEW_MESSAGE must have flags');
       return addFlagsForMessages(state, [action.message.id], action.message.flags);
+    }
 
     case EVENT_UPDATE_MESSAGE_FLAGS:
       return eventUpdateMessageFlags(state, action);