Extract out a source_span package from source_maps.

pkg/source_span/lib/src/colors.dart

Index: pkg/source_span/lib/src/colors.dart
diff --git a/pkg/docgen/test/multi_library_code/lib/test_lib2_foo.dart b/pkg/source_span/lib/src/colors.dart
similarity index 55%
copy from pkg/docgen/test/multi_library_code/lib/test_lib2_foo.dart
copy to pkg/source_span/lib/src/colors.dart
index 1b3d037c5bce3add4829e93b527bc5eb7f01f821..274fc92af617e0f7a06ab6c10a1d6ae95f042ef3 100644
--- a/pkg/docgen/test/multi_library_code/lib/test_lib2_foo.dart
+++ b/pkg/source_span/lib/src/colors.dart
@@ -2,14 +2,12 @@
 // 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');
-}