feature(is-registered): added a boolean method to indicate if a provider type or name is registered.

kiwi/lib/src/kiwi_container.dart

@@ -150,11 +150,17 @@ class KiwiContainer {
     _namedProviders.clear();
   }
 
+  /// Returns if an instance or builder of type [T] is registered.
+  ///
+  /// If an instance or builder of type [T] is registered with a name,
+  /// and the name is not passed to [isRegistered], returns false.
+  bool isRegistered<T>({String? name}) {
+    return (_namedProviders.containsKey(name) &&
+        _namedProviders[name]!.containsKey(T));
+  }
+
   void _setProvider<T>(String? name, _Provider<T> provider) {
-    final nameProviders = _namedProviders;
-    if (!silent &&
-        (nameProviders.containsKey(name) &&
-            nameProviders[name]!.containsKey(T))) {
+    if (!silent && isRegistered<T>(name: name)) {
       throw KiwiError(
           'The type `$T` was already registered${name == null ? '' : ' for the name `$name`'}');
     }

kiwi/test/kiwi_test.dart

@@ -393,6 +393,51 @@ void main() {
             'Not Registered KiwiError:\n\n\nFailed to resolve `int`:\n\nThe type `int` was not registered\n\nMake sure `int` is added to your KiwiContainer and rerun build_runner build\n(If you are using the kiwi_generator)\n\nWhen using Flutter, most of the time a hot restart is required to setup the KiwiContainer again.\n\n\n',
           )));
     });
+
+    test('checks that the instances are registered', () {
+      final scoped = KiwiContainer.scoped();
+
+      // Unnamed instances
+      expect(scoped.isRegistered<int>(), false);
+
+      scoped.registerInstance<int>(5);
+
+      expect(scoped.isRegistered<int>(), true);
+      expect(scoped.resolve<int>(), 5);
+
+      scoped.unregister<int>();
+
+      expect(scoped.isRegistered<int>(), false);
+      expect(
+          () => container.resolve<int>(),
+          throwsA(TypeMatcher<KiwiError>().having(
+            (f) => f.toString(),
+            'toString()',
+            'Not Registered KiwiError:\n\n\nFailed to resolve `int`:\n\nThe type `int` was not registered\n\nMake sure `int` is added to your KiwiContainer and rerun build_runner build\n(If you are using the kiwi_generator)\n\nWhen using Flutter, most of the time a hot restart is required to setup the KiwiContainer again.\n\n\n',
+          )));
+
+      // Named instances
+      expect(scoped.isRegistered<String>(name: 'named_string_instance'), false);
+
+      scoped.registerInstance<String>('random_string',
+          name: 'named_string_instance');
+
+      expect(scoped.isRegistered<String>(),
+          false); // [isRegistered] cannot be true if String it is named and is tested unnamed.
+      expect(scoped.isRegistered<String>(name: 'named_string_instance'), true);
+      expect(scoped.resolve<String>('named_string_instance'), 'random_string');
+
+      scoped.unregister<String>('named_string_instance');
+
+      expect(scoped.isRegistered<String>(), false);
+      expect(
+          () => container.resolve<String>(),
+          throwsA(TypeMatcher<KiwiError>().having(
+            (f) => f.toString(),
+            'toString()',
+            'Not Registered KiwiError:\n\n\nFailed to resolve `String`:\n\nThe type `String` was not registered\n\nMake sure `String` is added to your KiwiContainer and rerun build_runner build\n(If you are using the kiwi_generator)\n\nWhen using Flutter, most of the time a hot restart is required to setup the KiwiContainer again.\n\n\n',
+          )));
+    });
   });
 }