Implement hashCode on objects stored in a set or used as map keys.

dart/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 582 matching lines @@
 
   String toString() {
     return computeMessage();
   }
 
   bool operator==(other) {
     if (other is !Message) return false;
     return (kind == other.kind) && (toString() == other.toString());
   }
 
+  int get hashCode => throw new UnsupportedError('Message.hashCode');
+
   String slowToString(object) {
     if (object is SourceString) {
       return object.slowToString();
     } else {
       return object.toString();
     }
   }
 }
 
 class Diagnostic {
@@ skipping 20 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }