unreads [nfc]: Use for/in style instead of forEach

Sometimes we get feedback from a lint rule --
  https://dart.dev/tools/linter-rules/avoid_function_literals_in_foreach_calls
-- whose documentation recommends using for loops instead of
forEach.

That rule didn't fire on these instances; shrug; but this file has
had a mix of for loops and forEach loops, and it seems nicer to
consistently use one or the other.

Just in case there was a performance bug in a Map.forEach
implementation, I tested mark-as-read (by scrolling through the web
app) to see if we might get a free performance benefit there, where
the norm on my device (with a profile build) has been to take about
14ms per event:
  zulip#304 (comment)
But, unsurprisingly, it stayed at around 14ms as before.

lib/model/unreads.dart

@@ -287,14 +287,23 @@ class Unreads extends ChangeNotifier {
                     .add(messageId);
               }
             }
-            newlyUnreadInStreams.forEach((incomingStreamId, incomingTopics) {
-              incomingTopics.forEach((incomingTopic, incomingMessageIds) {
+            for (
+              final MapEntry(key: incomingStreamId, value: incomingTopics)
+              in newlyUnreadInStreams.entries
+            ) {
+              for (
+                final MapEntry(key: incomingTopic, value: incomingMessageIds)
+                in incomingTopics.entries
+              ) {
                 _addAllInStreamTopic(incomingMessageIds..sort(), incomingStreamId, incomingTopic);
-              });
-            });
-            newlyUnreadInDms.forEach((incomingDmNarrow, incomingMessageIds) {
+              }
+            }
+            for (
+              final MapEntry(key: incomingDmNarrow, value: incomingMessageIds)
+              in newlyUnreadInDms.entries
+            ) {
               _addAllInDm(incomingMessageIds..sort(), incomingDmNarrow);
-            });
+            }
         }
     }
     notifyListeners();
@@ -328,21 +337,21 @@ class Unreads extends ChangeNotifier {
   // TODO use efficient model lookups
   void _slowRemoveAllInStreams(Set<int> idsToRemove) {
     final newlyEmptyStreams = [];
-    streams.forEach((streamId, topics) {
+    for (final MapEntry(key: streamId, value: topics) in streams.entries) {
       final newlyEmptyTopics = [];
-      topics.forEach((topic, messageIds) {
+      for (final MapEntry(key: topic, value: messageIds) in topics.entries) {
         messageIds.removeWhere((id) => idsToRemove.contains(id));
         if (messageIds.isEmpty) {
           newlyEmptyTopics.add(topic);
         }
-      });
+      }
       for (final topic in newlyEmptyTopics) {
         topics.remove(topic);
       }
       if (topics.isEmpty) {
         newlyEmptyStreams.add(streamId);
       }
-    });
+    }
     for (final streamId in newlyEmptyStreams) {
       streams.remove(streamId);
     }
@@ -387,12 +396,12 @@ class Unreads extends ChangeNotifier {
   // TODO use efficient model lookups
   void _slowRemoveAllInDms(Set<int> idsToRemove) {
     final newlyEmptyDms = [];
-    dms.forEach((dmNarrow, messageIds) {
+    for (final MapEntry(key: dmNarrow, value: messageIds) in dms.entries) {
       messageIds.removeWhere((id) => idsToRemove.contains(id));
       if (messageIds.isEmpty) {
         newlyEmptyDms.add(dmNarrow);
       }
-    });
+    }
     for (final dmNarrow in newlyEmptyDms) {
       dms.remove(dmNarrow);
     }