Only support the new source_span spans in barback.

pkg/smoke/CHANGELOG.md

Index: pkg/smoke/CHANGELOG.md
diff --git a/pkg/smoke/CHANGELOG.md b/pkg/smoke/CHANGELOG.md
index a9140ae9ff2a9d2630241d47d9b863da908c402a..89d50befd8cf4595ca1dc5152cb365809013fd74 100644
--- a/pkg/smoke/CHANGELOG.md
+++ b/pkg/smoke/CHANGELOG.md
@@ -2,7 +2,10 @@
 
 This file contains highlights of what changes on each version of this package.
 
-#### Pub version 0.2.0+1
+#### Pub version 0.2.0+3
+  * Widen the constraint on barback.
+
+#### Pub version 0.2.0+2
   * Switch from `source_maps`' `Span` class to `source_span`'s `SourceSpan`
     class.