Fix FileSpan.context to include FileSpan.text.

lib/src/file.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.
 
 library source_span.file;
 
 import 'dart:math' as math;
 import 'dart:typed_data';
 
 import 'package:path/path.dart' as p;
@@ skipping 187 matching lines @@
   ///
   /// [end] is lazily generated from this to avoid allocating unnecessary
   /// objects.
   final int _end;
 
   Uri get sourceUrl => file.url;
   int get length => _end - _start;
   FileLocation get start => new FileLocation._(file, _start);
   FileLocation get end => new FileLocation._(file, _end);
   String get text => file.getText(_start, _end);
-
-  String get context {
-    var line = start.line;
-    return file.getText(file.getOffset(line),
-        line == file.lines - 1 ? null : file.getOffset(line + 1));
-  }
+  String get context => file.getText(file.getOffset(start.line),
+      end.line == file.lines - 1 ? null : file.getOffset(end.line + 1));
 
   FileSpan._(this.file, this._start, this._end) {
     if (_end < _start) {
       throw new ArgumentError('End $_end must come after start $_start.');
     } else if (_end > file.length) {
       throw new RangeError("End $_end must not be greater than the number "
           "of characters in the file, ${file.length}.");
     } else if (_start < 0) {
       throw new RangeError("Start may not be negative, was $_start.");
     }
@@ skipping 38 matching lines @@
     if (sourceUrl != other.sourceUrl) {
       throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
           " \"${other.sourceUrl}\" don't match.");
     }
 
     var start = math.min(this._start, other._start);
     var end = math.max(this._end, other._end);
     return new FileSpan._(file, start, end);
   }
 }