dart-mode has been split into two packages:
- dart-mode, Provides basic syntax highlighting and indentation.
- dart-server, Factors out the features from dart-mode.el that relied on
external executables, thus improving availability, reliability, testing and
development of dart-mode.el.
This commit removes the workarounds needed on older versions of `dart-mode` library, now that many of the fixes have landed.
This commit also fixes the integration with flycheck, as there is a new function used on the `develop` branch.