Generate a resolution error if a unary + is used for something other than numbers.

frog/leg/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.
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static final GENERIC = const MessageKind('#{1}');
 
@@ skipping 61 matching lines @@
       'redirecting constructor cannot have other initializers');
   static final SUPER_INITIALIZER_IN_OBJECT = const MessageKind(
       "'Object' cannot have a super initializer");
   static final DUPLICATE_SUPER_INITIALIZER = const MessageKind(
       'cannot have more than one super initializer');
   static final NO_MATCHING_CONSTRUCTOR = const MessageKind(
       'no matching constructor found');
   static final NO_CONSTRUCTOR = const MessageKind(
       '#{1} is a #{2}, not a constructor');
   static final FIELD_PARAMETER_NOT_ALLOWED = const MessageKind(
-      'A field parameter is only allowed in generative constructors');
+      'a field parameter is only allowed in generative constructors');
   static final INVALID_PARAMETER = const MessageKind(
-      "Cannot resolve parameter");
+      "cannot resolve parameter");
   static final NOT_INSTANCE_FIELD = const MessageKind(
       '#{1} is not an instance field');
+  static final EXPECTED_LITERAL_NUMBER = const MessageKind(
+      'expected a literal number after +');
 
   static final NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
       '#{1} cannot be used as compile-time constant');
 
   toString() => template;
 }
 
 class Message {
   final kind;
   final List arguments;
@@ skipping 42 matching lines @@
   String toString() => message.toString();
 }
 
 class CompileTimeConstantError {
   final Message message;
   CompileTimeConstantError.message(this.message);
   CompileTimeConstantError(MessageKind kind, List<Type> arguments)
     : message = new Message(kind, arguments);
   String toString() => message.toString();
 }