Stop running pub get at gclient sync time and fix build bugs

mojo/public/dart/third_party/barback/pubspec.yaml

Index: mojo/public/dart/third_party/barback/pubspec.yaml
diff --git a/mojo/public/dart/third_party/barback/pubspec.yaml b/mojo/public/dart/third_party/barback/pubspec.yaml
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+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+6
+
+author: "Dart Team <misc@dartlang.org>"
+homepage: http://github.com/dart-lang/barback
+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"