Fix sourcemap after changes in source_span

lib/src/source_map_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.
 
 library source_maps.source_map_span;
 
 import 'package:source_span/source_span.dart';
 
 /// A [SourceSpan] for spans coming from or being written to source maps.
 ///
@@ skipping 26 matching lines @@
 
 /// A wrapper aruond a [FileSpan] that implements [SourceMapSpan].
 class SourceMapFileSpan implements SourceMapSpan, FileSpan {
   final FileSpan _inner;
   final bool isIdentifier;
 
   SourceFile get file => _inner.file;
   FileLocation get start => _inner.start;
   FileLocation get end => _inner.end;
   String get text => _inner.text;
+  String get context => _inner.context;
   Uri get sourceUrl => _inner.sourceUrl;
   int get length => _inner.length;
 
   SourceMapFileSpan(this._inner, {this.isIdentifier: false});
 
   int compareTo(SourceSpan other) => _inner.compareTo(other);
   SourceSpan union(SourceSpan other) => _inner.union(other);
   FileSpan expand(FileSpan other) => _inner.expand(other);
   String message(String message, {color}) =>
       _inner.message(message, color: color);
   String toString() => _inner.toString()
       .replaceAll("FileSpan", "SourceMapFileSpan");
 }