upgraded code for nullsafety

lib/resolvers/resolver.dart

@@ -7,5 +7,5 @@ abstract class Resolver<T> {
      * Разрешает зависимость типа [T]
      * @return - возвращает объект типа [T]
      */
-  T resolve();
+  T? resolve();
 }

lib/resolvers/resolving_context.dart

@@ -6,11 +6,13 @@ import 'package:dart_di/resolvers/value_resolver.dart';
 
 class ResolvingContext<T> extends Resolver {
   /// Корневой резолвер
-  Resolver<T> get resolver => _resolver;
+  Resolver<T> get resolver {
+    return _resolver as Resolver<T>;
+  }
 
   DiContainer _container;
 
-  Resolver _resolver;
+  late Resolver _resolver;
 
   ResolvingContext(this._container);
 
@@ -21,7 +23,7 @@ class ResolvingContext<T> extends Resolver {
   @override
   T resolve() {
     _verify();
-    return _resolver?.resolve();
+    return _resolver.resolve();
   }
 
   /**
@@ -38,7 +40,7 @@ class ResolvingContext<T> extends Resolver {
      *  Создать резолвер значения
      */
   ResolvingContext<T> toValue<TImpl extends T>(T value) {
-    Resolver<TImpl> resolver = ValueResolver(value);
+    Resolver<TImpl> resolver = ValueResolver(value as TImpl);
     return toResolver<TImpl>(resolver);
   }
 

lib/resolvers/singelton_resolver.dart

@@ -2,12 +2,12 @@ import 'package:dart_di/resolvers/resolver.dart';
 
 class SingletonResolver<T> extends Resolver<T> {
   Resolver<T> _decoratedResolver;
-  T _value = null;
+  T? _value = null;
 
   SingletonResolver(this._decoratedResolver);
 
   @override
-  T resolve() {
+  T? resolve() {
     if (_value == null) {
       _value = _decoratedResolver.resolve();
     }