Report compile-time error instead of crashing when encountering bad parts.

pkg/front_end/lib/src/fasta/export.dart

Index: pkg/front_end/lib/src/fasta/export.dart
diff --git a/pkg/front_end/lib/src/fasta/export.dart b/pkg/front_end/lib/src/fasta/export.dart
index 2d4a83783068308de1c5436b3c8260c6fe1bb6ed..78723ce8ddf2b5bd015b6fb8655ab118aa6f91d2 100644
--- a/pkg/front_end/lib/src/fasta/export.dart
+++ b/pkg/front_end/lib/src/fasta/export.dart
@@ -17,7 +17,9 @@ class Export {
 
   final List<Combinator> combinators;
 
-  Export(this.exporter, this.exported, this.combinators, int charOffset);
+  final int charOffset;
+
+  Export(this.exporter, this.exported, this.combinators, this.charOffset);
 
   Uri get fileUri => exporter.fileUri;