DevFS.update only runs in Dart 2 mode when the generator parameter is
supplied. In Dart 2 mode, both mainPath and pathToReload are required
parameters; this patch marks them as such.

generator is required for running in Dart 2. All call sites other than tests already explicitly set this value.

Note the statements on line 510 and line 516 for why mainPath and pathToReload are required.