Emit a compile-time error when a named optional parameter has a library private name.

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index f0035ae53e19950059751fe6776a732df2977f36..7d5dcbf30e087af02b5bbb8efdbc1743bac34565 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -933,6 +933,13 @@ Use 'const #{name}' if possible.''');
       "Error: '#{value}' is not a valid Symbol name because it starts with "
       "'_'.");
 
+  static const MessageKind PRIVATE_NAMED_PARAMETER = const MessageKind(
+      "Error: Named optional parameter can't have a library private name.",
+      howToFix: "Try removing the '_' or making the parameter positional or "
+        "required.",
+      examples: const ["""foo({int _p}) {} main() => foo();"""]
+      );
+
   static const MessageKind UNSUPPORTED_LITERAL_SYMBOL = const MessageKind(
       "Internal Error: Symbol literal '##{value}' is currently unsupported.");