Remove barback from the repo.

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.
-version: 0.15.2+2
-
-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"