Extract out a source_span package from source_maps.

pkg/source_span/lib/src/colors.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.
 
-library test_lib2_foo;
+// Color constants used for generating messages.
+library source_span.colors;
 
-export 'test_lib2_bar.dart';
+const String RED = '\u001b[31m';
 
-class Foo {
+const String YELLOW = '\u001b[33m';
 
-}
+const String NONE = '\u001b[0m';
 
-main() {
-  print('hello world');
-}