Updating to work on Dart 1.4.0-dev.2

Pub in 1.4 is requiring barback 0.13 which has a breaking change in isPrimary (see https://codereview.chromium.org//223553008). This change allows DI to target either barback 0.11.1+ or 0.13.0, depending on pub version.

End result is that this allows DI to work on both stable and dev channel of Dart.