Our build is currently non-deterministic and therefor hard to reproduce because many of the external dependencies are being resolved during build time, so it's possible for a single sha to create many different build artifacts depending on when the build happens.

To fix this, we need to lock down all external dependencies.

This will include:

• npm dependencies (which should be locked down via npm-shrinkwrap.json)
• pub dependencies (which should be locked down via pubspec.lock file)
• dart sdk + dartium dependencies (which should be locked down by fetching a concrete version rather than whatever the latest version is

Once the dependencies are locked, we can use travis cache to cache the dependencies and speed up the build startup time just like we did in the angular.js repo.