[cfe] Correct PrivateNamedParameter problem message

Named parameters can't start with underscores whether they are optional or required.

Change-Id: Iaaeab80f74a691b4981148f06412a60ca115f73e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337883
Auto-Submit: Parker Lougheed <parlough@gmail.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/_fe_analyzer_shared/lib/src/messages/codes_generated.dart

@@ -12369,7 +12369,8 @@ const Code<Null> codePrivateNamedParameter = messagePrivateNamedParameter;
 const MessageCode messagePrivateNamedParameter = const MessageCode(
     "PrivateNamedParameter",
     analyzerCodes: <String>["PRIVATE_OPTIONAL_PARAMETER"],
-    problemMessage: r"""An optional named parameter can't start with '_'.""");
+    problemMessage:
+        r"""A named parameter can't start with an underscore ('_').""");
 
 // DO NOT EDIT. THIS FILE IS GENERATED. SEE TOP OF FILE.
 const Code<Null> codeRecordFieldsCantBePrivate =

pkg/front_end/messages.yaml

@@ -1995,7 +1995,7 @@ FunctionTypeDefaultValue:
   analyzerCode: DEFAULT_VALUE_IN_FUNCTION_TYPE
 
 PrivateNamedParameter:
-  problemMessage: "An optional named parameter can't start with '_'."
+  problemMessage: "A named parameter can't start with an underscore ('_')."
   analyzerCode: PRIVATE_OPTIONAL_PARAMETER
 
 NoFormals:

tests/language/parameter/initializer6_test.dart

@@ -11,8 +11,7 @@ class Foo {
   Foo.private({this._y = 77}) {}
   //                ^^
   // [analyzer] COMPILE_TIME_ERROR.PRIVATE_OPTIONAL_PARAMETER
-  //                ^
-  // [cfe] An optional named parameter can't start with '_'.
+  // [cfe] A named parameter can't start with an underscore ('_').
 }
 
 main() {