Make SourceSpanException.source a getter

lib/src/span_exception.dart

 // Copyright (c) 2014, 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.
 
 import 'span.dart';
 
 /// A class for exceptions that have source span information attached.
 class SourceSpanException implements Exception {
   /// A message describing the exception.
   final String message;
@@ skipping 14 matching lines @@
   /// it indicates that the text shouldn't be highlighted.
   String toString({color}) {
     if (span == null) return message;
     return "Error on " + span.message(message, color: color);
   }
 }
 
 /// A [SourceSpanException] that's also a [FormatException].
 class SourceSpanFormatException extends SourceSpanException
     implements FormatException {
-  final source;
+  final _source;
+
+  // Subclasses may narrow the type.
+  dynamic get source => _source;
 
   int get offset => span == null ? null : span.start.offset;
 
-  SourceSpanFormatException(String message, SourceSpan span, [this.source])
+  SourceSpanFormatException(String message, SourceSpan span, [this._source])
       : super(message, span);
 }