Allow some fields to be overridden in strong mode.

lib/src/span_with_context.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.dart';
 import 'utils.dart';
 
 /// A class that describes a segment of source text with additional context.
 class SourceSpanWithContext extends SourceSpanBase {
+  // This is a getter so that subclasses can override it.
   /// Text around the span, which includes the line containing this span.
-  final String context;
+  String get context => _context;
+  final String _context;
 
   /// Creates a new span from [start] to [end] (exclusive) containing [text], in
   /// the given [context].
   ///
   /// [start] and [end] must have the same source URL and [start] must come
   /// before [end]. [text] must have a number of characters equal to the
   /// distance between [start] and [end]. [context] must contain [text], and
   /// [text] should start at `start.column` from the beginning of a line in
   /// [context].
   SourceSpanWithContext(
-      SourceLocation start, SourceLocation end, String text, this.context)
+          SourceLocation start, SourceLocation end, String text, this._context)
       : super(start, end, text) {
     if (!context.contains(text)) {
       throw new ArgumentError(
           'The context line "$context" must contain "$text".');
     }
 
     if (findLineStart(context, text, start.column) == null) {
       throw new ArgumentError('The span text "$text" must start at '
           'column ${start.column + 1} in a line within "$context".');
     }
   }
 }