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

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

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 915 matching lines @@
 Hint: Using "new #{name}' may result in larger output.
 Use 'const #{name}' if possible.''');
 
   static const MessageKind STRING_EXPECTED = const MessageKind(
       "Error: Expected a 'String', but got an instance of '#{type}'.");
 
   static const MessageKind PRIVATE_IDENTIFIER = const MessageKind(
       "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.");
 
   static const MessageKind INVALID_SYMBOL = const MessageKind('''
 Error: '#{value}' is not a valid Symbol name because is not:
  * an empty String,
  * a user defined operator,
  * a qualified non-private identifier optionally followed by '=', or
  * a qualified non-private identifier followed by '.' and a user-defined '''
 "operator.");
@@ skipping 416 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, Map arguments, bool terse)
     : super(kind, arguments, terse);
 }