fix #29146, downgrade import error to warning in strong mode

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 3396 matching lines @@
    *
    * Parameters:
    * 0: the ambiguous name
    * 1: the name of the dart: library in which the element is found
    * 2: the name of the non-dart: library in which the element is found
    */
   static const StaticWarningCode CONFLICTING_DART_IMPORT =
       const StaticWarningCode(
           'CONFLICTING_DART_IMPORT',
           "Element '{0}' from SDK library '{1}' is implicitly hidden by '{2}'.",
-          "Try adding an explicit hide combinator.");
+          "Try adding an explicit hide combinator.",
+          false);
 
   /**
    * 7.2 Getters: It is a static warning if a class <i>C</i> declares an
    * instance getter named <i>v</i> and an accessible static member named
    * <i>v</i> or <i>v=</i> is declared in a superclass of <i>C</i>.
    *
    * Parameters:
    * 0: the name of the super class declaring a static member
    */
   static const StaticWarningCode
@@ skipping 1634 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;
 }