Throw an exception when invariant fails and report nice diagnostic.

dart/lib/compiler/implementation/util/util.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.
 
 library org_dartlang_compiler_util;
 
 import 'util_implementation.dart';
 
 part 'link.dart';
 
 /**
  * Tagging interface for classes from which source spans can be generated.
  */
 // TODO(johnniwinther): Find a better name.
+// TODO(ahe): How about "Bolt"?
 abstract class Spannable {}
+
+class SpannableAssertionFailure {
+  final Spannable node;
+  final String message;
+  SpannableAssertionFailure(this.node, this.message);
+
+  String toString() => 'compiler crashed.';
+}