Introduce span with line context

test/file_message_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';
 import 'package:source_span/src/colors.dart' as colors;
 
 main() {
   var file;
@@ skipping 44 matching lines @@
   test("works for a point span at the end of the file", () {
     expect(file.location(38).pointSpan().message("oh no"), equals("""
 line 3, column 12 of foo.dart: oh no
 zip zap zop
            ^"""));
   });
 
   test("works for a point span in an empty file", () {
     expect(new SourceFile("").location(0).pointSpan().message("oh no"),
         equals("""
-line 1, column 1: oh no
-
-^"""));

[Siggi Cherem (dart-lang), 2015/03/21 00:19:59]

would you consider this a breaking change? 

I could preserve the old behavior, but it is nice that if there is no context
(as in the case of an empty file), there is little value for showing this empty
line after all.

[nweiz, 2015/03/24 23:01:17]

No, I don't consider the specific format of [message] to be a guaranteed part of
the API.

[Siggi Cherem (dart-lang), 2015/03/25 00:33:25]

Turns out the fix for this was much easier than I thought, so I decided to keep
this unchanged for the time being.

+line 1, column 1: oh no"""));
   });
 
   test("works for a single-line file without a newline", () {
     expect(new SourceFile("foo bar").span(0, 7).message("oh no"),
         equals("""
 line 1, column 1: oh no
 foo bar
 ^^^^^^^"""));
   });
 
@@ skipping 13 matching lines @@
     });
 
     test("uses the given color if it's passed", () {
       expect(file.span(4, 7).message("oh no", color: colors.YELLOW), equals("""
 line 1, column 5 of foo.dart: oh no
 foo ${colors.YELLOW}bar${colors.NONE} baz
     ${colors.YELLOW}^^^${colors.NONE}"""));
     });
   });
 }