implemented resolvers

lib/resolvers/singelton_resolver.dart

@@ -0,0 +1,16 @@
+import 'package:dart_di/resolvers/resolver.dart';
+
+class SingletonResolver<T> extends Resolver<T> {
+  Resolver<T> _decoratedResolver;
+  T _value = null;
+
+  SingletonResolver(this._decoratedResolver);
+
+  @override
+  T resolve() {
+    if (_value == null) {
+      _value = _decoratedResolver.resolve();
+    }
+    return _value;
+  }
+}