Only support the new source_span spans in barback.

pkg/observe/CHANGELOG.md

Index: pkg/observe/CHANGELOG.md
diff --git a/pkg/observe/CHANGELOG.md b/pkg/observe/CHANGELOG.md
index e9cf1c4574ca81936a9d9913de46426530677d02..1f5adbc4f160827565bc193c3dc5346827d795a7 100644
--- a/pkg/observe/CHANGELOG.md
+++ b/pkg/observe/CHANGELOG.md
@@ -3,6 +3,9 @@
 This file contains highlights of what changes on each version of the observe
 package.
 
+#### Pub version 0.11.0+2
+  * Widen the constraint on barback.
+
 #### Pub version 0.11.0+1
   * Switch from `source_maps`' `Span` class to `source_span`'s `SourceSpan`
     class.