Add a dependency on collection to barback's pubspec.

pkg/barback/pubspec.yaml

 name: barback
 
 # Note! This version 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 version of this is upgraded, you *must* update that version
 # number in pub to stay in sync with this. New patch versions are considered
 # backwards compatible, and pub will allow later patch versions automatically.
 version: 0.12.0-dev
 
 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"
   source_maps: ">=0.9.0 <0.10.0"
   stack_trace: ">=0.9.1 <0.10.0"
+  collection: ">=0.9.1 <0.10.0"
 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"