[Build] Readd the 'clean:js' and 'clean:dart' tasks inside the 'build:js' and 'build:dart' tasks.
[Build] Hide the 'browserify' and 'uglify' tasks inside the 'buid:js' task. Remove the --js and --dart options of the 'lint', 'build' and 'watch' tasks. lib/dart-sdk/bin MUST be in your PATH for 'build:dart' to work.