Allow some fields to be overridden in strong mode.

lib/src/span.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 'location.dart';
 import 'span_mixin.dart';
 
 /// A class that describes a segment of source text.
 abstract class SourceSpan implements Comparable<SourceSpan> {
   /// The start location of this span.
@@ skipping 40 matching lines @@
   /// 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 message(String message, {color});
 }
 
 /// A base class for source spans with [start], [end], and [text] known at
 /// construction time.
 class SourceSpanBase extends SourceSpanMixin {
-  final SourceLocation start;
-  final SourceLocation end;
-  final String text;
+  // These fields go through getters so that subclasses can override them.
 
-  SourceSpanBase(this.start, this.end, this.text) {
+  SourceLocation get start => _start;
+  final SourceLocation _start;
+
+  SourceLocation get end => _end;
+  final SourceLocation _end;
+
+  String get text => _text;
+  final String _text;
+
+  SourceSpanBase(this._start, this._end, this._text) {
     if (end.sourceUrl != start.sourceUrl) {
       throw new ArgumentError("Source URLs \"${start.sourceUrl}\" and "
           " \"${end.sourceUrl}\" don't match.");
     } else if (end.offset < start.offset) {
       throw new ArgumentError('End $end must come after start $start.');
     } else if (text.length != start.distance(end)) {
       throw new ArgumentError('Text "$text" must be ${start.distance(end)} '
           'characters long.');
     }
   }
 }