Add a note to barback indicating that version 0.15.1 should be skipped.

pkg/barback/pubspec.yaml

 name: barback
 
 # Note! This version number is referenced directly in the pub source code in
 # lib/src/barback.dart. Pub implicitly places a version constraint on barback to
 # ensure users only select a version of barback that works with their current
 # version of pub.
 #
 # When the minor or patch version of this is upgraded, you *must* update that
 # version constraint in pub to stay in sync with this.
+#
+# Additional note: a stable version of pub was erroneously released with a
+# constraint on barback that includes 0.15.1 before version 0.15.1 was actually
+# released. As such, next time the patch version is bumped it should skip 0.15.1
+# and go directly to 0.15.2.
 version: 0.15.0+1
 
 author: "Dart Team <misc@dartlang.org>"
 homepage: http://www.dartlang.org
 description: >
   An asset build system.
 
   Given a set of input files and a set of transformations (think compilers,
   preprocessors and the like), will automatically apply the appropriate
   transforms and generate output files. When inputs are modified, automatically
   runs the transforms that are affected.
 
   Runs transforms asynchronously and in parallel when possible to maximize
   responsiveness.
 dependencies:
   path: ">=0.9.0 <2.0.0"
   pool: ">=1.0.0 <2.0.0"
   source_span: ">=1.0.0 <2.0.0"
   stack_trace: ">=0.9.1 <2.0.0"
   collection: ">=0.9.1 <2.0.0"
 dev_dependencies:
   scheduled_test: ">=0.9.0 <0.11.0"
   unittest: ">=0.9.0 <0.10.0"
 environment:
   sdk: ">=1.0.1 <2.0.0"