Implement constant maps.

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 104 matching lines @@
   static final UNUSED_LABEL = const MessageKind(
       'unused label #{1}');
   static final INVALID_CONTINUE = const MessageKind(
       'target of continue is not a loop or switch case');
   static final INVALID_USE_OF_SUPER = const MessageKind(
       'super not allowed here');
 
   static final NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
       'not a compile-time constant');
 
+  static final KEY_NOT_A_STRING_LITERAL = const MessageKind(
+      'map-literal key not a string literal');
+
   static final NO_SUCH_LIBRARY_MEMBER = const MessageKind(
       '#{1} has no member named #{2}');
 
   static final CANNOT_INSTANTIATE_INTERFACE = const MessageKind(
       "cannot instantiate interface '#{1}'");
 
   static final CANNOT_INSTANTIATE_TYPEDEF = const MessageKind(
       "cannot instantiate typedef '#{1}'");
 
   static final NO_DEFAULT_CLASS = const MessageKind(
@@ skipping 59 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();
 }