Throw input error instead of parser error to avoid crashing.

pkg/front_end/lib/src/fasta/source/stack_listener.dart

Index: pkg/front_end/lib/src/fasta/source/stack_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/source/stack_listener.dart b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
index 1438a2e19e66e2ec12d5ecef0647ab9c2aac4ab6..0e03ae66ec42dd0d9141cf5eae139191bcf978ce 100644
--- a/pkg/front_end/lib/src/fasta/source/stack_listener.dart
+++ b/pkg/front_end/lib/src/fasta/source/stack_listener.dart
@@ -232,4 +232,9 @@ abstract class StackListener extends Listener {
     super.handleRecoverableError(token, kind, arguments);
     debugEvent("Error: ${recoverableErrors.last}");
   }
+
+  @override
+  Token handleUnrecoverableError(Token token, ErrorKind kind, Map arguments) {
+    throw inputError(uri, token.charOffset, "$kind $arguments");
+  }
 }