Upgrade to the new test runner.

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:test/test.dart';
 import 'package:source_span/source_span.dart';
 import 'package:source_span/src/colors.dart' as colors;
 
 main() {
   var file;
   setUp(() {
     file = new SourceFile("""
 foo bar baz
 whiz bang boom
 zip zap zop
@@ skipping 75 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}"""));
     });
   });
 }