Fix FileSpan.context to include FileSpan.text.

test/file_test.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 'package:unittest/unittest.dart';
 import 'package:source_span/source_span.dart';
 
 main() {
   var file;
   setUp(() {
@@ skipping 235 matching lines @@
       expect(span.end, equals(location));
       expect(span.text, isEmpty);
     });
   });
 
   group("FileSpan", () {
     test("text returns a substring of the source", () {
       expect(file.span(8, 15).text, equals("baz\nwhi"));
     });
 
+    test("context contains the span's text", () {
+      var span = file.span(8, 15);
+      expect(span.context.contains(span.text), isTrue);
+      expect(span.context, equals('foo bar baz\nwhiz bang boom\n'));
+    });
+
     group("union()", () {
       var span;
       setUp(() {
         span = file.span(5, 12);
       });
 
       test("works with a preceding adjacent span", () {
         var other = file.span(0, 5);
         var result = span.union(other);
         expect(result.start, equals(other.start));
@@ skipping 94 matching lines @@
         var other = file.span(7, 10);
         expect(span.expand(other), equals(span));
       });
 
       test("works with an external overlapping span", () {
         var other = file.span(0, 16);
         expect(span.expand(other), equals(other));
       });
     });
   });
-}
+}