Fix flakiness in hot_reload_test.dart (#158271)

The test was immediately checking the contents of stdout after the daemon indicated that the hot reload had completed. This could cause a race since the reloaded code may not have had time to execute.

Fixes flutter/flutter#158245

Author: bkonyi

packages/flutter_tools/test/integration.shard/hot_reload_test.dart

@@ -69,11 +69,21 @@ void main() {
 
   testWithoutContext('newly added code executes during hot reload', () async {
     final StringBuffer stdout = StringBuffer();
-    final StreamSubscription<String> subscription = flutter.stdout.listen(stdout.writeln);
+    final Completer<void> completer = Completer<void>();
+    final StreamSubscription<String> subscription = flutter.stdout.listen((String e) {
+      stdout.writeln(e);
+      // If hot reload properly executes newly added code, the 'RELOAD WORKED' message should
+      // be printed before 'TICK 2'. If we don't wait for some signal that the build method
+      // has executed after the reload, this test can encounter a race.
+      if (e.contains('((((TICK 2))))')) {
+        completer.complete();
+      }
+    });
     await flutter.run();
     project.uncommentHotReloadPrint();
     try {
       await flutter.hotReload();
+      await completer.future;
       expect(stdout.toString(), contains('(((((RELOAD WORKED)))))'));
     } finally {
       await subscription.cancel();