implemented toResolver

lib/resolvers/resolving_context.dart

@@ -25,8 +25,8 @@ class ResolvingContext<T> extends Resolver {
      * пользовательский резолвер
      */
   ResolvingContext<T> toResolver<TImpl extends T>(Resolver<TImpl> resolver) {
-    // TODO: implement toResolver
+    _resolver = resolver;
-    throw UnimplementedError();
+    return this;
   }
 
   /**

test/di_container_test.dart

@@ -0,0 +1,35 @@
+import 'package:dart_di/di_container.dart';
+import 'package:dart_di/resolvers/resolver.dart';
+import 'package:test/test.dart';
+import 'package:mockito/mockito.dart';
+
+void main() {
+  group('Without parent', () {
+    test('Container bind<T> throws state error if it\'s already has resolver',
+        () {
+      final container = new DiContainer();
+      container.bind<int>().toResolver(_makeResolver(5));
+
+      expect(() => container.bind<int>().toResolver(_makeResolver(3)),
+          throwsA(isA<StateError>()));
+    });
+  });
+}
+
+ResolverMock<T> _makeResolver<T>(T expectedValue) {
+  final resolverMock = new ResolverMock<T>();
+  when(resolverMock.resolve()).thenReturn(expectedValue);
+  return resolverMock;
+}
+
+class ResolverMock<T> extends Mock implements Resolver<T> {}
+
+abstract class A {}
+
+class B implements A {}
+
+class DependOnA {
+  final A a;
+
+  DependOnA(this.a) : assert(a != null);
+}