Be able to override the "pub build" command

[Gérald Reinhart]

By default pub build is launched after the pub get, it can be usefull to use another command: for instance "pub build --mode=debug" or "/app/dart-sdk/bin/dart build.dart"

See discussion https://plus.google.com/u/0/+SethLadd/posts/7hMnq5ZyvXb?cfem=1

[Seth Ladd]

De-duping this issue.