Fix FileSpan.context to include FileSpan.text.

test/file_test.dart

Index: test/file_test.dart
diff --git a/test/file_test.dart b/test/file_test.dart
index 114a17e44ba116d3895cffd270f0665c88d2f894..c27c1f64fe876ed97097be71ffc64c5aa37c8745 100644
--- a/test/file_test.dart
+++ b/test/file_test.dart
@@ -253,6 +253,12 @@ zip zap zop""", url: "bar.dart").span(10, 11);
       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(() {
@@ -367,4 +373,4 @@ zip zap zop""", url: "bar.dart").span(10, 11);
       });
     });
   });
-}
+}