Fix generated server code to be strong mode compliant

pkg/analysis_server/tool/spec/api.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Data structures representing an API definition, and visitor base classes
  * for visiting those data structures.
  */
 library api;
 
@@ skipping 34 matching lines @@
       : this.experimental = experimental ?? false;
 }
 
 /**
  * Base class for visiting the API definition.
  */
 abstract class ApiVisitor<T> {
   /**
    * Dispatch the given [type] to the visitor.
    */
-  T visitTypeDecl(TypeDecl type) => type.accept(this);
+  T visitTypeDecl(TypeDecl type) => type.accept(this) as T;
   T visitTypeEnum(TypeEnum typeEnum);
   T visitTypeList(TypeList typeList);
   T visitTypeMap(TypeMap typeMap);
   T visitTypeObject(TypeObject typeObject);
   T visitTypeReference(TypeReference typeReference);
 
   T visitTypeUnion(TypeUnion typeUnion);
 }
 
 /**
@@ skipping 435 matching lines @@
   /**
    * The field that is used to disambiguate this union
    */
   final String field;
 
   TypeUnion(this.choices, this.field, dom.Element html, {bool experimental})
       : super(html, experimental);
 
   accept(ApiVisitor visitor) => visitor.visitTypeUnion(this);
 }