Improve the message for default_list_constructor

Change-Id: Id2514bd9aab667939396b26f677577ff3b6695fe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150692
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson

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

@@ -1547,8 +1547,10 @@ class CompileTimeErrorCode extends AnalyzerErrorCode {
   // var l = List.generate(3, (i) => i);
   // ```
   static const CompileTimeErrorCode DEFAULT_LIST_CONSTRUCTOR =
-      CompileTimeErrorCode('DEFAULT_LIST_CONSTRUCTOR',
-          "Calling the default 'List' constructor causes an error.",
+      CompileTimeErrorCode(
+          'DEFAULT_LIST_CONSTRUCTOR',
+          "The default 'List' constructor is not available when null safety is "
+              "enabled.",
           correction: "Try using a list literal, 'List.filled' or "
               "'List.generate'.");
 

pkg/analyzer/tool/diagnostics/diagnostics.md

@@ -1379,7 +1379,7 @@ class C {
 
 ### default_list_constructor
 
-_Calling the default 'List' constructor causes an error._
+_The default 'List' constructor is not available when null safety is enabled._
 
 #### Description