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

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

 // Copyright (c) 2016, 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 fasta.export;
 
 import 'builder/builder.dart' show Builder, LibraryBuilder;
 
 import 'combinator.dart' show Combinator;
 
 class Export {
   /// The library that is exporting [exported];
   final LibraryBuilder exporter;
 
   /// The library being exported.
   final LibraryBuilder exported;
 
   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;
 
   bool addToExportScope(String name, Builder member) {
     if (combinators != null) {
       for (Combinator combinator in combinators) {
         if (combinator.isShow && !combinator.names.contains(name)) return false;
         if (combinator.isHide && combinator.names.contains(name)) return false;
       }
     }
     return exporter.addToExportScope(name, member);
   }
 }