Remove cancel and make crash exit with code 253.

sdk/lib/_internal/compiler/implementation/util/util.dart

Index: sdk/lib/_internal/compiler/implementation/util/util.dart
diff --git a/sdk/lib/_internal/compiler/implementation/util/util.dart b/sdk/lib/_internal/compiler/implementation/util/util.dart
index dc4474ea1843e9e361d32b577a37f1466a457cad..b24cc1f0d7a5627eb0916212b2dc9084731e3603 100644
--- a/sdk/lib/_internal/compiler/implementation/util/util.dart
+++ b/sdk/lib/_internal/compiler/implementation/util/util.dart
@@ -29,9 +29,18 @@ class _SpannableSentinel implements Spannable {
   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;