flags [nfc]: Fix some read-only vs. read-write object types in reducer

We've been breaching here the read-only markers on the properties
of FlagsState and of its per-flag subtrees.  The reason Flow hasn't
complained seems to be related to the use of computed properties
here; when we make these `flag` variables have a union type that
enumerates the actual expected flags, rather than just `string`,
it would start complaining.  Get ahead of that by fixing the issue.

As a tiny bonus, we make fewer lookups into these `newState` objects.
(Though this code could use some deeper data structures / algorithms
improvements than that.  Some other time.)

Author: gnprice

src/chat/flagsReducer.js

@@ -1,6 +1,7 @@
 /* @flow strict-local */
 import invariant from 'invariant';
 
+import type { ReadWrite } from '../generics';
 import type { PerAccountApplicableAction, FlagsState, Message } from '../types';
 import {
   REGISTER_COMPLETE,
@@ -13,6 +14,9 @@ import {
 } from '../actionConstants';
 import { deeperMerge } from '../utils/misc';
 
+type ReadWriteFlagsState = $Rest<ReadWrite<$ObjMap<FlagsState, <V>(V) => ReadWrite<V>>>, { ... }>;
+type ReadWritePerFlagState = $Values<ReadWriteFlagsState>;
+
 const initialState = {
   read: {},
   starred: {},
@@ -40,14 +44,14 @@ const addFlagsForMessages = (
   /* $FlowFixMe[incompatible-exact] - We should ignore flags from the server
      that we don't already know about. After all, we can't have any code
      intending to do anything with them. */
-  const newState: FlagsState = {};
+  const newState: ReadWriteFlagsState = {};
 
   flags.forEach(flag => {
-    newState[flag] = { ...(state[flag] || {}) };
-
+    const perFlag: ReadWritePerFlagState = { ...(state[flag] || {}) };
     messages.forEach(message => {
-      newState[flag][message] = true;
+      perFlag[message] = true;
     });
+    newState[flag] = perFlag;
   });
 
   return {
@@ -84,14 +88,12 @@ const processFlagsForMessages = (
   /* $FlowFixMe[incompatible-exact] - We should ignore flags from the server
      that we don't already know about. After all, we can't have any code
      intending to do anything with them. */
-  const newState: FlagsState = {};
+  const newState: ReadWriteFlagsState = {};
   messages.forEach(msg => {
     (msg.flags || []).forEach(flag => {
       if (!state[flag] || !state[flag][msg.id]) {
-        if (!newState[flag]) {
-          newState[flag] = {};
-        }
-        newState[flag][msg.id] = true;
+        const perFlag: ReadWritePerFlagState = newState[flag] || (newState[flag] = {});
+        perFlag[msg.id] = true;
         stateChanged = true;
       }
     });