Remove unnecessary type assertions

Author: renggli

lib/src/definition/resolve.dart

@@ -38,7 +38,7 @@ Parser<R> _dereference<R>(Parser<R> parser, Map<Parser, Parser> mapping) {
   final references = <ResolvableParser<R>>{};
   while (parser is ResolvableParser<R>) {
     if (mapping.containsKey(parser)) {
-      return mapping[parser]! as Parser<R>;
+      return mapping[parser] as Parser<R>;
     } else if (!references.add(parser)) {
       throw StateError('Recursive references detected: $references');
     }

lib/src/reflection/transform.dart

@@ -30,5 +30,5 @@ Parser<R> transformParser<R>(Parser<R> parser, TransformationHandler handler) {
       }
     }
   }
-  return mapping[parser]! as Parser<R>;
+  return mapping[parser] as Parser<R>;
 }