Only allow 'void' as a return type.

lib/compiler/implementation/warnings.dart

Index: lib/compiler/implementation/warnings.dart
===================================================================
--- lib/compiler/implementation/warnings.dart	(revision 12312)
+++ lib/compiler/implementation/warnings.dart	(working copy)
@@ -242,11 +242,14 @@
       "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.");