Fix null safety error in inspector service extensions taking a variable number of args. (#68661)

* Fix null safety error in inspector service extensions taking variable numbers of args.

Fixes https://github.com/flutter/flutter/issues/68627.
Also fix all inspector tests that do not rely on golden image functionality
so that they run with --enable-experiment=non-nullable. This verifies there
is test coverage to ensure there isn't a regression.

* Remove uses of dynamic.

packages/flutter/lib/src/widgets/widget_inspector.dart

@@ -880,17 +880,18 @@ mixin WidgetInspectorService {
     registerServiceExtension(
       name: name,
       callback: (Map<String, String> parameters) async {
-        const String argPrefix = 'arg';
         final List<String> args = <String>[];
-        parameters.forEach((String name, String value) {
-          if (name.startsWith(argPrefix)) {
-            final int index = int.parse(name.substring(argPrefix.length));
-            if (index >= args.length) {
-              args.length = index + 1;
-            }
-            args[index] = value;
+        int index = 0;
+        while (true) {
+          final String name = 'arg$index';
+          if (parameters.containsKey(name)) {
+            args.add(parameters[name]!);
+          } else {
+            break;
           }
-        });
+          index++;
+        }
+        assert(index == parameters.length);
         return <String, Object?>{'result': await callback(args)};
       },
     );

packages/flutter/test/widgets/widget_inspector_test_utils.dart

@@ -30,7 +30,7 @@ class TestWidgetInspectorService extends Object with WidgetInspectorService {
   }
 
   List<Map<Object, Object?>> getEventsDispatched(String eventKind) {
-    return eventsDispatched.putIfAbsent(eventKind, () => <Map<Object, Object>>[]);
+    return eventsDispatched.putIfAbsent(eventKind, () => <Map<Object, Object?>>[]);
   }
 
   Iterable<Map<Object, Object?>> getServiceExtensionStateChangedEvents(String extensionName) {