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

pkg/analyzer/lib/src/error/codes.dart

Index: pkg/analyzer/lib/src/error/codes.dart
diff --git a/pkg/analyzer/lib/src/error/codes.dart b/pkg/analyzer/lib/src/error/codes.dart
index 52d12b3a9dad4bfada7d4704d3bd16d7cee111e1..9a04fd2e9e8cffc3c647587e1e9c567c5863f9b8 100644
--- a/pkg/analyzer/lib/src/error/codes.dart
+++ b/pkg/analyzer/lib/src/error/codes.dart
@@ -3414,7 +3414,8 @@ class StaticWarningCode extends ErrorCode {
       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