Add token invariant to DiagnosticReporter

pkg/compiler/lib/src/diagnostics/invariant.dart

 // Copyright (c) 2012, 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.
 
 library dart2js.diagnostics.invariant;
 
 import 'spannable.dart';
 
 /**
  * If true, print a warning for each method that was resolved, but not
@@ skipping 30 matching lines @@
  */
 bool invariant(Spannable spannable, var condition, {var message: null}) {
   // TODO(johnniwinther): Use [spannable] and [message] to provide better
   // information on assertion errors.
   if (spannable == null) {
     throw new SpannableAssertionFailure(CURRENT_ELEMENT_SPANNABLE,
         "Spannable was null for invariant. Use CURRENT_ELEMENT_SPANNABLE.");
   }
   if (condition is Function){
     condition = condition();
+    if (condition is String) {
+      message = condition;
+      condition = false;
+    }
   }
   if (!condition) {
     if (message is Function) {
       message = message();
     }
     throw new SpannableAssertionFailure(spannable, message);
   }
   return true;
 }
 
 typedef void InternalErrorFunction(Spannable location, String message);