Turns out that ignoring all error lines that match the empty string is a
poor way to go.
Also, we have to update all the example packages now too, since we
analyze them. So just have travis use our update script.
Also, remove flutter_tools' old travis stuff. It's now part of a bigger
repo.
Also, make travis use the dev Dart SDK, since we need the new analyzer.
Stable is way too out of date, e.g. it still complains about libraries
not having names and mixins using 'super', and the strong mode hints are
even more aggressive than on dev.
