test: Generate random increasing streamId for example data by default

This allows `eg.stream` to be used in contexts where it's important
that the stream IDs don't collide or even that they're increasing,
without the test having to explicitly give specific stream IDs when
no other fact about them is relevant.

Some tests are dependent on the stream IDs being static. Those are
updated to use `eg.staticStreamId` explicitly.

Author: Khader-1

test/example_data.dart

@@ -147,6 +147,19 @@ final User fourthUser  = user(fullName: 'Fourth User', email: 'fourth@example');
 // Streams and subscriptions.
 //
 
+/// A fresh stream ID, from a random but always strictly increasing sequence.
+int _nextStreamId() => (_lastStreamId += 1 + Random().nextInt(10));
+int _lastStreamId = 100;
+
+const staticStreamId = 123;
+
+/// Construct an example stream.
+///
+/// If stream ID `streamId` is not given, it will be generated from a random
+/// but increasing sequence.
+/// Use an explicit `streamId` only if the ID needs to correspond to some
+/// other data in the test, or if the IDs need to increase in a different order
+/// from the calls to [stream].
 ZulipStream stream({
   int? streamId,
   String? name,
@@ -163,7 +176,7 @@ ZulipStream stream({
   int? streamWeeklyTraffic,
 }) {
   return ZulipStream(
-    streamId: streamId ?? 123, // TODO generate example IDs
+    streamId: streamId ?? _nextStreamId(),
     name: name ?? 'A stream', // TODO generate example names
     description: description ?? 'A description', // TODO generate example descriptions
     renderedDescription: renderedDescription ?? '<p>A description</p>', // TODO generate random
@@ -301,7 +314,7 @@ StreamMessage streamMessage({
   int? timestamp,
   List<MessageFlag>? flags,
 }) {
-  final effectiveStream = stream ?? _stream();
+  final effectiveStream = stream ?? _stream(streamId: staticStreamId);
   // The use of JSON here is convenient in order to delegate parts of the data
   // to helper functions.  The main downside is that it loses static typing
   // of the properties as we're constructing the data.  That's probably OK

test/model/message_list_test.dart

@@ -38,7 +38,7 @@ void main() {
 
   /// Initialize [model] and the rest of the test state.
   Future<void> prepare({Narrow narrow = const CombinedFeedNarrow()}) async {
-    final stream = eg.stream();
+    final stream = eg.stream(streamId: eg.staticStreamId);
     subscription = eg.subscription(stream);
     store = eg.store();
     await store.addStream(stream);
@@ -248,13 +248,13 @@ void main() {
   });
 
   test('MessageEvent, not in narrow', () async {
-    final stream = eg.stream(streamId: 123);
+    final stream = eg.stream();
     await prepare(narrow: StreamNarrow(stream.streamId));
     await prepareMessages(foundOldest: true, messages:
       List.generate(30, (i) => eg.streamMessage(stream: stream)));
 
     check(model).messages.length.equals(30);
-    final otherStream = eg.stream(streamId: 234);
+    final otherStream = eg.stream();
     await store.handleEvent(MessageEvent(id: 0,
       message: eg.streamMessage(stream: otherStream)));
     checkNotNotified();
@@ -651,7 +651,7 @@ void main() {
     // doesn't need to exercise the different reasons that messages don't.
 
     const timestamp = 1693602618;
-    final stream = eg.stream();
+    final stream = eg.stream(streamId: eg.staticStreamId);
     Message streamMessage(int id) =>
       eg.streamMessage(id: id, stream: stream, topic: 'foo', timestamp: timestamp);
     Message dmMessage(int id) =>
@@ -732,7 +732,7 @@ void main() {
 
     const t1 = 1693602618;
     const t2 = t1 + 86400;
-    final stream = eg.stream();
+    final stream = eg.stream(streamId: eg.staticStreamId);
     Message streamMessage(int id, int timestamp, User sender) =>
       eg.streamMessage(id: id, sender: sender,
         stream: stream, topic: 'foo', timestamp: timestamp);
@@ -773,8 +773,8 @@ void main() {
     });
 
     test('stream messages match just if same stream/topic', () {
-      final stream0 = eg.stream(streamId: 123);
-      final stream1 = eg.stream(streamId: 234);
+      final stream0 = eg.stream();
+      final stream1 = eg.stream();
       final messageAB = eg.streamMessage(stream: stream0, topic: 'foo');
       final messageXB = eg.streamMessage(stream: stream1, topic: 'foo');
       final messageAX = eg.streamMessage(stream: stream0, topic: 'bar');

test/model/message_test.dart

@@ -34,7 +34,7 @@ void main() {
 
   /// Initialize [store] and the rest of the test state.
   Future<void> prepare({Narrow narrow = const CombinedFeedNarrow()}) async {
-    final stream = eg.stream();
+    final stream = eg.stream(streamId: eg.staticStreamId);
     subscription = eg.subscription(stream);
     store = eg.store();
     await store.addStream(stream);

test/widgets/message_list_test.dart

@@ -52,7 +52,7 @@ void main() {
     UnreadMessagesSnapshot? unreadMsgs,
   }) async {
     addTearDown(testBinding.reset);
-    streams ??= subscriptions ??= [eg.subscription(eg.stream())];
+    streams ??= subscriptions ??= [eg.subscription(eg.stream(streamId: eg.staticStreamId))];
     await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot(
       streams: streams, subscriptions: subscriptions, unreadMsgs: unreadMsgs));
     store = await testBinding.globalStore.perAccount(eg.selfAccount.id);