Only allow 'void' as a return type.

lib/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.
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static const GENERIC = const MessageKind('#{1}');
 
@@ skipping 224 matching lines @@
   static const ILLEGAL_FINAL_METHOD_MODIFIER = const MessageKind(
       "Error: cannot have final modifier on method.");
 
   static const ILLEGAL_CONSTRUCTOR_MODIFIERS = const MessageKind(
       "Error: illegal constructor modifiers: #{1}.");
 
   static const PARAMETER_NAME_EXPECTED = const MessageKind(
       "Error: parameter name expected.");
 
   static const CANNOT_RESOLVE_GETTER = const MessageKind(
-      'Cannot resolve getter.');
+      'cannot resolve getter.');
 
   static const CANNOT_RESOLVE_SETTER = const MessageKind(
-      'Cannot resolve setter.');
+      'cannot resolve setter.');
+
+  static const VOID_NOT_ALLOWED = const MessageKind(
+      'type void is only allowed in a return type.');
 
   static const COMPILER_CRASHED = const MessageKind(
       "Error: The compiler crashed when compiling this element.");
 
   static const PLEASE_REPORT_THE_CRASH = const MessageKind('''
 The compiler is broken.
 
 When compiling the above element, the compiler crashed. It is not
 possible to tell if this is caused by a problem in your program or
 not. Regardless, the compiler should not crash.
@@ skipping 78 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);
 }