Add strong mode error for mixins defining conflicting private names (issue 28809)

pkg/analyzer/lib/src/error/codes.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.
 
 library analyzer.src.error.codes;
 
 import 'package:analyzer/error/error.dart';
 
 export 'package:analyzer/src/analysis_options/error/option_codes.dart';
 export 'package:analyzer/src/dart/error/hint_codes.dart';
@@ skipping 2042 matching lines @@
    */
   static const CompileTimeErrorCode PREFIX_IDENTIFIER_NOT_FOLLOWED_BY_DOT =
       const CompileTimeErrorCode(
           'PREFIX_IDENTIFIER_NOT_FOLLOWED_BY_DOT',
           "The name '{0}' refers to an import prefix, so it must be followed "
           "by '.'.",
           "Try correcting the name to refer to something other than a prefix, or "
           "renaming the prefix.");
 
   /**
+   * It is an error for a mixin to add a private name that conflicts with a
+   * private name added by a superclass or another mixin.
+   */
+  static const CompileTimeErrorCode PRIVATE_COLLISION_IN_MIXIN_APPLICATION =
+      const CompileTimeErrorCode(
+          'PRIVATE_COLLISION_IN_MIXIN_APPLICATION',
+          "The private name {0}, defined by {1}, conflicts with the same name "
+          "defined by {2}.",
+          "Try removing {1} from the 'with' clause.");
+
+  /**
    * 6.2.2 Optional Formals: It is a compile-time error if the name of a named
    * optional parameter begins with an '_' character.
    */
   static const CompileTimeErrorCode PRIVATE_OPTIONAL_PARAMETER =
       const CompileTimeErrorCode('PRIVATE_OPTIONAL_PARAMETER',
           "Named optional parameters can't start with an underscore.");
 
   /**
    * 12.1 Constants: It is a compile-time error if the value of a compile-time
    * constant expression depends on itself.
@@ skipping 2884 matching lines @@
    * created from the optional [correction] template.
    */
   const StrongModeCode(ErrorType type, String name, String message,
       [String correction])
       : type = type,
         super('STRONG_MODE_$name', message, correction);
 
   @override
   ErrorSeverity get errorSeverity => type.severity;
 }