Throw an exception when invariant fails and report nice diagnostic.

dart/lib/compiler/implementation/util/util.dart

Index: dart/lib/compiler/implementation/util/util.dart
diff --git a/dart/lib/compiler/implementation/util/util.dart b/dart/lib/compiler/implementation/util/util.dart
index ef3b0f63967d080d320c22d8fb7b1e38a6c5a660..b39bd461920b19bda11a3c59cc85923cc0dc0f8a 100644
--- a/dart/lib/compiler/implementation/util/util.dart
+++ b/dart/lib/compiler/implementation/util/util.dart
@@ -12,4 +12,13 @@ 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.';
+}