Report a compile-time error when a setter has not exactly one formal parameter.

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;
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
@@ skipping 201 matching lines @@
 
   static const MISSING_ARGUMENTS_TO_ASSERT = const MessageKind(
       "missing arguments to assert");
 
   static const GETTER_MISMATCH = const MessageKind(
       "Error: setter disagrees on: #{1}.");
 
   static const SETTER_MISMATCH = const MessageKind(
       "Error: getter disagrees on: #{1}.");
 
+  static const ILLEGAL_SETTER_FORMALS = const MessageKind(
+      "Error: a setter must have exactly one argument.");
+
   static const NO_STATIC_OVERRIDE = const MessageKind(
       "Error: static member cannot override instance member '#{1}' of '#{2}'.");
 
   static const NO_STATIC_OVERRIDE_CONT = const MessageKind(
       "Info: this is the instance member that cannot be overridden "
       "by a static member.");
 
   static const CANNOT_OVERRIDE_FIELD_WITH_METHOD = const MessageKind(
       "Error: method cannot override field '#{1}' of '#{2}'.");
 
@@ skipping 200 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }