Allow some fields to be overridden in strong mode.

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 {
+  // This is a getter so that subclasses can override it.
   /// A message describing the exception.
-  final String message;
+  String get message => _message;
+  final String _message;
 
+  // This is a getter so that subclasses can override it.
   /// The span associated with this exception.
   ///
   /// This may be `null` if the source location can't be determined.
-  final SourceSpan span;
+  SourceSpan get span => _span;
+  final SourceSpan _span;
 
-  SourceSpanException(this.message, this.span);
+  SourceSpanException(this._message, this._span);
 
   /// Returns a string representation of [this].
   ///
   /// [color] may either be a [String], a [bool], or `null`. If it's a string,
   /// it indicates an ANSII terminal color escape that should be used to
   /// highlight the span's text. If it's `true`, it indicates that the text
   /// should be highlighted using the default color. If it's `false` or `null`,
   /// 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 {
+  // This is a getter so that subclasses can override it.
+  dynamic get source => _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])
       : super(message, span);
 }