Remove cancel and make crash exit with code 253.

sdk/lib/_internal/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 dart2js.util;
 
 import "dart:collection";
 import 'util_implementation.dart';
 import 'characters.dart';
 
@@ skipping 11 matching lines @@
 abstract class Spannable {}
 
 class _SpannableSentinel implements Spannable {
   final String name;
 
   const _SpannableSentinel(this.name);
 
   String toString() => name;
 }
 
+/// Sentinel spannable used to mark that diagnostics should point to the
+/// current element. Note that the diagnostic reporting will fail if the current
+/// element is `null`.
 const Spannable CURRENT_ELEMENT_SPANNABLE =
     const _SpannableSentinel("Current element");
 
+/// Sentinel spannable used to mark that there might be no location for the
+/// diagnostic. Use this only when it is not an error not to have a current
+/// element.
+const Spannable NO_LOCATION_SPANNABLE =
+    const _SpannableSentinel("No location");
+
 class SpannableAssertionFailure {
   final Spannable node;
   final String message;
   SpannableAssertionFailure(this.node, this.message);
 
   String toString() => 'Assertion failure'
                        '${message != null ? ': $message' : ''}';
 }
 
 /**
@@ skipping 245 matching lines @@
   buffer.write(string);
 }
 
 int computeHashCode(part1, [part2, part3, part4, part5]) {
   return (part1.hashCode
           ^ part2.hashCode
           ^ part3.hashCode
           ^ part4.hashCode
           ^ part5.hashCode) & 0x3fffffff;
 }