Upgrade to the new test runner.

test/span_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 span;
   setUp(() {
     span = new SourceSpan(
         new SourceLocation(5, sourceUrl: "foo.dart"),
         new SourceLocation(12, sourceUrl: "foo.dart"),
         "foo bar");
@@ skipping 295 matching lines @@
     test("a different source URL isn't equal", () {
       var other = new SourceSpan(
           new SourceLocation(5, sourceUrl: "bar.dart"),
           new SourceLocation(12, sourceUrl: "bar.dart"),
           "foo bar");
 
       expect(span, isNot(equals(other)));
     });
   });
 }