[flutter_tools] Simplify `flutter test` internals (#74176)

packages/integration_test/test/binding_fail_test.dart

@@ -61,13 +61,18 @@ Future<Map<String, dynamic>> _runTest(String scriptPath) async {
   final String testResults = (await process.stdout
           .transform(utf8.decoder)
           .expand((String text) => text.split('\n'))
-          .map((String line) {
+          .map<dynamic>((String line) {
             try {
-              return jsonDecode(line) as Map<String, dynamic>;
+              return jsonDecode(line);
             } on FormatException {
               // Only interested in test events which are JSON.
             }
           })
+          .expand<Map<String, dynamic>>((dynamic json) {
+            return json is List<dynamic>
+                ? json.cast()
+                : <Map<String, dynamic>>[json as Map<String, dynamic>];
+          })
           .where((Map<String, dynamic> testEvent) =>
               testEvent != null && testEvent['type'] == 'print')
           .map((Map<String, dynamic> printEvent) => printEvent['message'] as String)