In the task 'modules/build.dart/pubspec' we do copy the pubspec to the build folder and launch pub get when the pubspec is modified.

It might happen that a pubspec depends on a pubspec that has not been copied yet, see https://travis-ci.org/angular/angular/builds/39469889#L227. Here, benchmark depends on facade but the pubspec of facade is not yet copied in the build folder which make the build fail.